1911 Encyclopædia Britannica/Insectivora

by Richard Lydekker

INSECTIVORA, an order of non-volant placental mammals of small size, with a dentition adapted to an insect-diet. In nearly all cases these creatures are nocturnal, and the majority are terrestrial, many burrowing in the ground, although a few are arboreal and others aquatic. They have plantigrade or partially plantigrade feet, that is to say, they apply the whole or the greater portion of the soles to the ground when walking; and there are generally five toes, each terminating in a claw, and the first never being opposable to the others in either the fore or hind limb. A full series of differentiated teeth, including temporary or deciduous milk-molars, is developed, and the cheek-teeth have distinct roots and are crowned with sharp cusps, which in some instances are three in number and arranged in a triangle. Very frequently the number of the teeth is the typical forty-four, arranged as i. 3/3, c. 1/1, p. 4/4, m. 3/3, but occasionally there is a fourth pair of molars, while the incisors may be reduced to two pairs above and one below, and the canine is frequently like an incisor or a premolar. The skull is of a primitive type, often with vacuities on the palate, as in marsupials, with a small brain-chamber, and the tympanic bone generally ring-like instead of forming a bladder-shaped bulla; except in the African Potamogale, clavicles, or collar-bones, are always present; the humerus generally has a perforation on the inner side of its lower extremity; and a centrale bone is usually present in the carpus. In the brain the smooth hemispheres are so short as to leave the cerebellum and sometimes even the corpora quadrigemina exposed. The uterus is two-horned; the placenta, so far as known, is deciduate and discoidal; the testes are abdominal or inguinal; and the teats usually numerous. The body in several instances is covered with sharp spines in place of hair.

The great majority of the Insectivora are nocturnal in their habits, and their whole structure indicates an extremely low grade of organisation, fully as low as that of marsupials. It is noteworthy that the dentition in several of the groups approximates to that of the extinct mammals of the Jurassic epoch (see Marsupialia), and exhibits more or less distinctly the primitive tritubercular type. Although the past history of the group is very imperfectly known, it seems probable that the Insectivora are nearly related to the original primitive mammalian stock. Indeed, it has been stated that were it not for the apparently advanced type of placenta, they might easily be regarded as the little modified descendants of the ancestors of most other mammals. Probably they are in some way related to the creodont carnivores (see Creodonta), but if, as has been suggested, the latter are akin to the primitive ungulates, the connexion would seem to be less close than has been sometimes supposed.

Representatives of this order are found throughout the temperate and tropical parts of both hemispheres, with the exception of South America (where only a few shrews have effected an entrance from the north) and Australia, and exhibit much variety both in organization and in habit. The greater number are cursorial, but some (Talpa, Chrysochloris, Oryzorictes) are burrowing, others (Limnogale, Potamogale, Nectogale, Myogale) aquatic, and some (Tupaiidae) arboreal. To the great majority the term insectivorous is applicable, although Potamogale is said to feed on fish, and the moles live chiefly on worms. Notwithstanding the nature of their food, much variety prevails in the form and number of the teeth, and while in many cases the division into incisors, canines, premolars and molars may be readily traced, in others, forming the great majority of the species, such as the shrews, this is difficult.

In most cases the brain-cavity is of small relative capacity, and in no instance is the brain-case elevated to any considerable extent above the face-line. The facial part of the skull is generally much produced, and the premaxillary and nasal bones well developed; but the cheek, or zygomatic arch, is usually slender or deficient, the latter being the case in most of the species, and post-orbital processes of the frontals are found only in the Tupaiidae and Macroscelididae. The number of dorsal vertebrae varies from 13 in Tupaia to 19 in Centetes, of lumbar from 3 in Chrysochloris to 6 in Talpa and Sorex, and of caudal from the rudimentary vertebrae of Centetes to the 40 or more well-developed ones of Microgale.

The breast-bone, or sternum, is variable, but generally narrow, bilobate in front and divided into segments. The shoulder-girdle presents extreme adaptive modifications in the mole, in relation to the use of the fore-limbs in burrowing; but in the golden moles the fore-arm and fore-foot alone become specially modified. In Macroscelides the bones of the fore-arm are united at their lower ends, but in all other Insectivora the radius and ulna are distinct. The fore-foot has generally five digits; but in Rhynchocyon and in one species of Oryzorictes the first toe is absent, and in the moles it is extremely modified. The femur has, in most species, a prominent ridge below the greater trochanter presenting the characters of a third trochanter. In Tupaia, Centetes, Hemicentetes, Ericulus and Solenodon the tibia and fibula are distinct, but in most other genera united. The hind-foot consists usually of five digits (rarely four by reduction of the first), and in some, as in the leaping species (Macroscelides, Rhynchocyon), the tarsal bones are elongated. The form of the pelvis, and especially of the symphysis pubis, varies within certain limits, so that while in the Tupaiidae and Macroscelididae there is a long symphysis, in the Erinaceidae, Centetidae and Potamogalidae it is short, and in the Soricidae, Talpidae and Chrysochloridae there is none.

Owing to the similarity in the character of the food, the truly insectivorous species, forming more than nine-tenths of the order, present little variety in the structure of the digestive organs. The stomach is a simple, thin-walled sac; sometimes as in Centetes, with the pyloric and oesophageal openings close together; the intestinal canal has much the same calibre throughout, and varies from three (in the shrews) to twelve times (in the hedgehogs) the length of the head and body. In the arboreal Tupaia and the allied Macroscelididae, which probably feed on vegetable substances as well as insects most of the species possess a caecum. The liver is deeply divided into lobes, the right and left lateral being cut off by deep fissures; both the caudate and Spigelian lobes are generally well developed, and the gall-bladder, usually large and globular, is placed on the middle of the posterior surface of the right central lobe.

All the members of the order appear to be highly prolific, the number of young varying from two to eight in the hedgehog, and from twelve to twenty-one in the tenrec. The position of the milk-glands and the number of teats vary greatly. In Solenodon there is a single pair of post-inguinal teats, but in most species these organs range from the thorax to the abdomen, varying from two pairs in Gymnura to twelve in the tenrec. In the golden moles the thoracic and inguinal teats are lodged in deep cut-shaped depressions.

Scent-glands exist in many species. In most shrews they occur on the sides of the body at a short distance behind the axilla, and their exudation is probably protective, as few carnivorous animals will eat their dead bodies. In both species of Gymnura and in Potamogale large pouches are situated on each side of the rectum, and discharge their secretions by ducts, opening in the first-named genus in front of and in the latter within the margin of the vent. In the tenrec similarly situated glands discharge by pores opening at the bottom of deep pits.

The skin is thin, but in many species lined with well-developed muscles, which are probably more developed in hedgehogs than in any other mammals. In this family and in the tenrec most of the species are protected by spines implanted in the skin-muscle, or panniculus carnosus.

The Insectivora may be divided into two groups, according to the degree of development of the union between the two halves of the pelvis. The first group is characterized by the full development of this union, both pubis and ischium entering into the symphysis. The tympanum remains as Tree-Shrews. a ring within an auditory bulla; the orbit is either surrounded by bone, or separated from the hinder part of the skull by a post-orbital process of the frontal; the upper molars have broad 5-cusped crowns with a W-shaped pattern; and the intestine is generally furnished with a caecum. The first family of this group is the Tupaiidae, represented by the tree-shrews, or tupaias, of the Indo-Malay countries, characterized by the complete bony ring round the eye-socket, the freedom of the fibula from the tibia in the hind-limb, and the absence of any marked elongation of the tarsus. The dental formula is i. 2/3, c. 1/1, p. 3/3, m. 3/3, total 38. In appearance and habits tree-shrews are extremely like squirrels, although they differ, of course, in toto as regards their dentition. A large number of species are included as the typical genus Tupaia, which ranges from north-eastern India to the great Malay Islands. In these animals the tail has a fringe of long hairs on opposite sides throughout its length. In the pen-tailed, tree-shrew (Ptilocercus lowii), fig. 1, the only representative of its genus, and a native of Sumatra, Borneo and the Malay Peninsula, the fringes of long hair are confined to the terminal third of the tail. There are also differences in the skulls of the two genera. A third genus, Urogale, represented by U. cylindrura of the mountains of Mindanao, in the Philippines, and U. everetti, of Borneo, has been established for the round-tailed tupaias, in which the tail is uniformly short-haired, and the second upper incisor and the lower canines are unusually large, the third lower incisor being proportionately small, and also erect, while the second upper incisor resembles a canine. (See Tree-Shrew.)

Fig. 1. Pen-tailed Tree-Shrew (Ptilocercus lowii).

In Africa the tupaias are apparently represented by the jumping-shrews, or elephant-shrews (so called from their elongated muzzles), constituting the family Macroscelididae. From the Tupaiidae the members of this family are readily distinguished by the fact that the socket of the eye, in place of having a complete bony ring, is separated from the hinder part of the skull merely by a post-orbital process of the frontal bone, and also by the more or less marked elongation of the tarsus or lower portion of the hind-limb; another feature being the union of the lower ends of the tibia and fibula. As indicated by one of their names, the members of the group leap after the fashion of gerbils, or jerboas, and hence walk much more on their toes than the majority of the order. In the typical genus Macroscelides, which ranges all over Africa and has numerous specific representatives, the dental formula is i. 3/3, c. 1/1, p. 4/4, m. 2/2 or 3, total 40 or 42; while there are five toes to each foot, and the lower ends of the radius and ulna are united. In Petrodromus (fig. 2) of East Africa, there are only four front-toes, and the hairs on the lower part of the tail form stiff bristles, with swollen tips; the dental formula being the same as that of those species of Macroscelides as have only two lower molars. A further reduction of the number of the digits takes place in the long-nosed jumping-shrews of the genus Rhynchocyon, which are larger animals with a much longer snout, only four toes to each foot, and a dental formula of i. 1 or 0/3, c. 1/1, p. 4/4, m. 2/2, total 36 or 34. Some of the species, all of which are East African, differ from the members of the typical genus by the deep rufous brown instead of olive-grey colour of their coat. (See Jumping-Shrew.)

In the second group, which includes all the other members of the order, the pelvic symphysis is either lacking or formed merely by the epiphyses of the pubes; the orbit and temporal region of the skull are confluent; and, except in the Talpidae and Chrysochloridae, the tympanum is ring-like, the tympanic cavity being formed by the alisphenoid and basisphenoid bones. The upper molars are triconodont, being either of the typical or a modified form of what is known as the tritubercular sectorial type. There is no caecum.

The first representatives of this group are the moles, or Talpidae, in which the lower ends of the tibia and fibula are united (fig. 3, t, fb), there is a descent of the testes, the tympanum forms a bladder-like bulla, the zygomatic, or cheek-arch, although slender, is complete, there is no pelvic symphysis, the upper Moles. molars are five-cusped, and the first upper incisor is simple, and the lower vertical. In habits the majority of the family are burrowing, but a few are aquatic; and all feed on animal substances. The distribution is limited to the temperate regions of Europe, Asia and North America.

Throughout the family the eyes are minute, and in some species are covered with skin; the ears are short and hidden in the fur; and the fore-limbs are generally more or less modified for digging.

Fig. 2.—Peter’s Jumping-Shrew (Petrodromus tetradactylus).

The true moles of the genus Talpa are the typical representatives of the first subfamily, or Talpinae, in which the clavicle (fig. 3, cl.) and humerus (h) are very short and broad, while there is an additional sickle-like bone (fc) on the inner side of the fore-foot. In Talpa itself the first upper incisor is but little larger than the second, the fore-foot is very broad, and the dental formula is i. 2/2, c. 1/1 or 0, p. 3/3, 3/4, or 4/4, m. 3/3. There are about a dozen species, all confined to the Old World. The variation in the dental formula of some of the best known of these is as follows:—

i. 3/3, c. 1/0, p. 4/4, m. 3/3 × 2 (T. wogura, robusta).
i. 3/3, c. 1/1, p. 4/4, m. 3/3 × 2 (T. europaea, caeca, romana, longirostris, micrura).
i. 3/3, c. 1/1, p. 3/4, m. 3/3 × 2 (T. leucura, leptura).
i. 3/3, c. 1/1, p. 3/3, m. 3/3 × 2 (T. moschata).

Except in T. europaea, the eyes are covered by a membrane. In T. micrura the short tail is concealed by the fur. T. europaea extends from England to Japan.

T. caeca and T. romana are found south of the Alps, the remaining species are all Asiatic, two only—T. micrura and T. leucura—occurring south of the Himalaya.

The genus may be split up into subgenera corresponding with the above table; these subdivisions being sometimes accorded full generic rank. For instance the Japanese T. wogura and the Siberian T. robusta are often referred to under the ill-sounding titles of Mogera wogura and M. robusta.

Fig. 3.—Skeleton of Mole (Talpa europaea) (lower jaw removed to show base of skull).
c, Calcaneum. c.h, Clavicular articulation of the humerus. cl,Clavicle. e.c, External condyle of humerus. f,Femur. fb,Fibula. fc,Falciform bone (radial sesamoid). h, Humerus. i.c,Internal condyle of humerus. il,Left iliac bone. i.p,Ramus of the ilium and pubis. is,Ischium. l.d,Ridge of insertion of latissimus dorsi muscle. l.t,Lesser trochanter. m,Manubrium sterni.	o,Fourth hypapophysial sesamoid ossicle. ol, Olecranon. p,Pubic bone widely separated from that of the opposite side. pa,Patella. p.m, Ridge for insertion of pectoralis major muscle. pt, Pectineal eminence. r,Radius. rb, First rib. s,Plantar sesamoid ossicle corresponding to the radial sesamoid (os falciform) in the manus. sc,Scapula. s.h,Scapular articulation of the humerus. t,Tibia. u,Ulna.

Referring more fully to the European species, it may be mentioned that the mole exhibits in its organization perfect adaptation to its mode of life. In the structure of the skeleton striking departures from the typical mammalian forms are noticeable. The first sternal bone is so much produced as to extend forward as far as a vertical line from the second cervical vertebra, carrying with it the very short almost quadrate clavicles, which are articulated with its anterior extremity and externally with the humeri, being also connected ligamentously with the scapula. The fore-limbs are thus brought opposite the sides of the neck, and from this position a threefold advantage is derived:—in the first place, as this is the narrowest part of the body, they add little to the width, which, if increased, would lessen the power of movement in a confined space; secondly this position allows of a longer fore-limb than would otherwise be possible, and so increases its lever power; and, thirdly, although the entire limb is relatively short, its anterior position enables the animal, when burrowing, to thrust the claws so far forward as to be in a line with the end of the muzzle, the importance of which is evident. Posteriorly, we find the hind-limbs removed out of the way by approximation of the hip-joints to the centre line of the body. This is effected by inward curvature of the innominate bones at the acetabulum to such an extent that they almost meet in the centre, while the pubic bones are widely separated behind. The shortness of the fore-limb is due to the humerus, which, like the clavicle, is so reduced in length as to present the appearance of a flattened X-shaped bone, with prominent ridges and deep depressions for the attachments of powerful muscles. Its upper extremity presents two rounded prominences; the smaller, the true head of the bone, articulates as usual with the scapula; the larger, which is the external tuberosity rounded off, forms a separate joint with the end of the clavicle. This double articulation gives the rigidity necessary to support the great lateral pressure sustained by the fore-limb in excavating. The bones of the fore-leg are normal, but those of the fore-foot are flattened and laterally expanded. The great width of the fore-foot is also partly due to the presence of a peculiar bone on the inner side of the palm and articulating with the wrist.

The muscles acting on these modified limbs are homologous with those of cursorial insectivora, differing only in their relative development. The tendon of the biceps traverses a long bony tunnel, formed by the expansion of the margin of the bicipital groove for the insertion of the pectoralis major muscle; the anterior division of the latter muscle is unconnected with the sternum, extending across as a band between the humeri, and co-ordinating the motions of the fore-limbs. The teres major and latissimus dorsi muscles are of immense size, inserted into the prominent ridge below the pectoral attachment, and are the principal agents in the excavating action of the limb. The cervical muscles connecting the slender scapulae, and through them the fore-limbs, with the centre line of the neck and with the occiput are large, and the ligamentum nuchae between them is ossified. The latter condition appears to be due to the prolongation forwards of the sternum, preventing flexion of the head downwards; and accordingly, the normal office of the ligament being lost, it ossifies, and affords a fixed point for the origins of the superficial cervical muscles.

The skull is long, with slender zygomatic arches; the nasal bones are strong and early become united, and in front of them the nostrils are continued forwards in tubes formed of thick cartilage, the septum between which becomes partially or wholly ossified beneath. There are 7 cervical, 13 dorsal, 6 lumbar, 6 sacral and 10-12 caudal vertebrae; of the dorsal and lumbar there may be one more or less. The sacral vertebrae are united by their expanded and compressed spinous processes, and all the others, with the exception of the cervical, are closely and solidly articulated together, so as to support the powerful propulsive and fossorial actions of the limbs. The upper incisors are simple chisel-edged teeth; the canine is long and two-rooted; then follow three subequal conical premolars, and a fourth, much larger, and like a canine; these are succeeded by three molars with W-shaped cusps. In the lower jaw the three incisors on each side are slightly smaller, and slant more forwards; close behind them is a tooth which, though like them, must, from its position in front of the upper canine, be considered as the canine; behind it, but separated by an interval, is a large double-rooted conical tooth, the first premolar; the three following premolars are like the corresponding teeth above, but smaller, and are succeeded, as above, by the three molars. See Mole.

Fig. 4.—Russian Desman (Myogale moschata).

In the other members of the Talpinae, which are North American, the first upper incisor is much taller than the second. They include the curious star-nosed mole (Condylura cristata), which has the typical series of 44 teeth and a series of fleshy appendages round the extremity of the snout; the species known as Scapanus townsendi and Parascalops americanus, each representing a genus by itself, and characterized by the absence of nasal appendages and the presence of only two pairs of lower incisors; and, finally, Scalops aquaticus, in which the dentition is further reduced by the loss of the lower canine, the total number of teeth thus being forty.

Forming a transition to the subfamily Myogalinae, in which the clavicle and humerus are typically of normal form, and there is no sickle-shaped bone in the fore-foot, is the Chinese mole (Scaptonyx piscicauda), characterized by having the clavicle and humerus of the true mole-type, but the foot like that of the under-mentioned Urotrichus. The relative proportions of the first and second upper incisors are also as in Talpa, but there are only two pairs of lower incisors.

Among the more typical Myogalinae, mention may be made of Dymecodon pilirostris, from Japan, representing a genus by itself; nearly allied to which are the shrew-moles, as represented by the small and long-tailed Urotrichus of Japan, with incisors 2/1 and premolars 4/3, and U. (Neurotrichus) gibbsi of North America, in which the premolars are 3/4. A still more interesting form is the Tibetan Uropsilus soricipes, a non-burrowing species, with the external appearance of a shrew combined with the skull of a mole, the feet being much narrower than in Urotrichus, and the dental formula i. 2/1, c. 1/1, p. 3/3, m. 3/3.

The typical representatives of the subfamily are the two European desmans, Myogale moschata and M. pyrenaica, which are aquatic in habits and have the feet webbed and the full series of 44 teeth. The former is by far the largest member of the whole family, its total length being about 16 in. Its long proboscis-like snout projects far beyond the margin of the upper lip; the toes are webbed as far as the bases of the claws; and the long scaly tail is laterally flattened, forming a powerful instrument of propulsion when swimming. This species inhabits the banks of streams and lakes in south-east Russia, where its food consists of various aquatic insects. M. pyrenaica, living in a similar manner in the Pyrenees, is much smaller, has a cylindrical tail, and a relatively long snout.

The Shrew-mice, or, shortly, shrews (Soricidae), are closely related to the Talpidae, with which they are connected by means of some of the subfamily Myogalinae. They are, however, distinguished by the ring-like tympanic, the incompleteness of the zygomatic arch, the tubercular-sectorialShrews. type of upper molar, the two-cusped first upper incisor, and the forward direction of the corresponding lower tooth. As a rule they are terrestrial, but a few are aquatic.

 

Fig. 5.—Skull and Dentition of a Shrew-mouse (Sorex-verae-pacis); i, first incisors; c in the upper jaw is the canine; and p-m the three premolars, behind which are the three molars; in the lower jaw c is the second incisor, and p the single premolar.

The dentition (fig. 5) is characteristic, and affords one of the chief means of classifying this exceedingly difficult group of mammals. There are no lower canines, and always six functional teeth on each side of the lower jaw, but in some rare instances an additional rudimentary tooth is squeezed in between two of the others. The first pair of teeth in each jaw differ from the rest; in the upper jaw they are hooked and have a more or less pronounced basal cusp; in the lower jaw they are long and project horizontally forwards, sometimes with an upward curve at the tip. Behind the first upper incisor comes a variable number of small teeth, of which, when all are developed, the first two are incisors, the third the canine, and the next two premolars; behind these, again, are four larger teeth, of which the front one is the last premolar, while the other three are molars. Thus we have in the typical genus Sorex (fig. 5) the dental formula i. 3/2, c. 1/0, p. 3/1, m. 3/3, total 32, or twenty upper and twelve lower teeth. The lower formula, as already stated, is constant, but the number of the upper series varies from the above maximum of twenty to a minimum of fourteen in Diplomesodon and Anurosorex, in which the formula is i. 2, c. 1, p. 1, m. 3. From the relation of the fourth upper tooth to the premaxillo-maxillary suture it has been supposed that shrews, like many polyprotodont marsupials, have four pairs of upper incisors; but this is improbable, and the formula is accordingly here taken to follow the ordinary placental type.

Shrews may be divided into two sections, according as to whether the teeth are tipped with brownish or reddish or are wholly white, the former group constituting the Soricinae and the latter the Crocidurinae.

In the red-tipped group is the typical genus Sorex, which ranges over Europe and Asia north of the Himalaya Mountains to North America. There are twenty upper teeth with the formula given above, the ears are well developed, the tail is long and evenly haired, and the aperture of the generative organs in at least one of the sexes is distinct from the vent. The common shrew-mouse (Sorex araneus) has a distribution co-extensive with that of the genus in the Old World, and the North American S. richardsoni can scarcely be regarded as more than a local race. A few species, such as Sorex hydrodomus of Alaska and S. palustris of the United States, have fringes of long hairs on the feet, and are aquatic in habit. The latter has been made the type of the genus Neosorex, but such a distinction, according to Dr J. E. Dobson, is unnecessary. The same authority likewise rejects the separation of the North American S. bendirei as Atophyrax, remarking that this species is an inhabitant of marshy land, and appears to present many characters intermediate between S. palustris and the terrestrial species of the genus, differing from the former in the absence of well-defined fringes to the digits, but agreeing with it closely in dentition, in the large size of the infra-orbital foramen, and in the remarkable shortness of the angular process of the lower jaw. In India and Burma the place of Sorex is taken by Soriculus, in which the upper teeth are generally 18, although rarely 20, and the generative organs have an opening in common with the vent after the fashion of the monotreme mammals. The latter feature occurs in the North American Blarina, which is characterized by the truncation of the upper part of the ear and the short tail, the number of upper teeth being 20 or 18. Another American genus, Notiosorex, in which the ear is well developed and the tail medium, has only 16 upper teeth. From all the rest of the red-toothed group the water-shrew, Neomys (or Crossopus) fodiens, of Europe and northern Asia, differs by the fringe of long hairs on the lower surface of the tail; the number of upper teeth being 18.

In the white-toothed, or crocidurine, group, the small African genus Myosorex, which has either 18 or 20 upper teeth, includes long-tailed and large-eared species in which the aperture of the generative organs and the vent, although close together, are yet distinct. In the musk-shrews (Crocidura), on the other hand, which are common to Europe, Asia and Africa, the reproductive organs and the alimentary canal discharge into a common cloaca, the long tail is sparsely covered with long and short hairs, there are anal glands secreting a strong musky fluid, and the number of upper teeth is 16 or 18. Diplomesodon pulchellus of the Kirghiz steppes, has, on the other hand, only 14 upper teeth, and is further characterized by the moderately long tail and the hairy soles of the hind-feet. Another genus is represented by the Tibetan Anurosorex squamipes, which has the same dental formula, but a mole-like form, rudimentary tail and scaly hind-soles. Lastly, we have two Asiatic mountain aquatic species, Chimarrogale himalayaca of the Himalayas and Nectogale elegans of Tibet, which have fringed tails like the European water-shrew, and 16 upper teeth, the former characterized by the small but perfect external ears, and the latter (fig. 6) by the absence of the ears and presence of adhesive disks on the feet.

It will be seen that the red- and the white-toothed series have parallel representative forms, which may indicate that the division of the family into the two groups is one based rather on convenience than on essential differences. See Shrew.

From the shrews, the hedgehogs and gymnuras, or rat-shrews, collectively forming the family Erinaceidae, differ structurally by the broader ring made by the tympanic, the complete zygomatic arch, the five-cusped broad upper molars, and the presence of a short pubic symphysis. At the present day they are an exclusively Old World group.

Fig. 6.—The Tibetan Water-shrew (Nectogale elegans).

The typical group, or Erinaceinae, is represented only by the hedgehogs, with the one genus Erinaceus, easily recognized by their spiny coats, and further characterized by the rudimentary tail, the presence of vacuities in the palate, and the broad pelvis. Hedgehogs (Erinaceus) have the dental formula Hedgehogs. i. 2/2, c. 1/1, p. 3/2, m. 3/3, and are represented by over a score of species, distributed throughout Europe, Africa and the greater part of Asia, but unknown in Madagascar, Ceylon, Burma, Siam, the Malay countries, and, of course, Australia. All the species resemble one another in the armour of spines covering the upper surface and sides of the body; and all possess the power of rolling themselves up into the form of a ball protected on all sides by these spines, the skin of the back being brought downwards and inwards over the head and tail so as to include the limbs by the action of special muscles.

Curiously enough the European hedgehog (E. europaeus) is the most aberrant species, differing from all the rest in the peculiarly-shaped and single-rooted third upper incisor and first premolar (fig. 7, A), and in its very coarse harsh fur. The dentition of the long-eared Indian E. grayi (fig. 7, B) may, on the other hand, be considered characteristic of all the other species, the only important differences being found in the variable size and position of the second upper premolar, which is very small, external and deciduous in the Indian E. micropus and E. pictus. The former species, limited to South India, is further distinguished by the absence of the jugal bone. Of African species, E. diadematus, with long frontal spines, is probably the commonest, and E. albiventris has been made the type of a separate genus on account of the total absence of the first front-toe. See Hedgehog.

The members of the second subfamily, Gymnurinae, are more or less rat-like animals, confined to the Malay countries, and easily distinguished from the hedgehogs by the absence of spines among the fur and the well-developed tail. They also lack vacuities in the palate, and have a long and narrow pelvis. Rat-shrew. The typical representative of the family is the greater rat-shrew, or greater gymnura (Gymnura rafflesi) a creature which may be compared to a giant shrew, and whose colour is partly black and partly white, although a uniformly pale-coloured race. (G. r. alba) inhabits Borneo. In common with the next genus, it has the full series of 44 teeth; and its range extends from Tenasserim and the Malay Peninsula to Sumatra and Borneo, the island individuals being stated to be considerably larger than those from the mainland. In this species the length of the tail is about three-fourths that of the head and body; but in the lesser rat-shrew (Hylomys suillus), ranging from Burma and the Malay Peninsula to Java and Sumatra, the former dimension is only about one-sixth of the latter. In the Philippines the group is represented by Podogymnura truei, distinguished from the other genera by the great elongation of the hind-foot, the tail being likewise long. There are only three pairs of premolars in each jaw.

Fig. 7.—Fore-part of Skulls of Common Hedgehog (Erinaceus europaeus),
A, and Gray’s Hedgehog (E. grayi), B, much enlarged.

Fig. 8.—The Insectivorous Otter (Potagmogale velox).

In the remaining families of the Insectivora the tibia and fibula may be either separated or united at the lower end; there is no descent of the testes, except in Solenodon; a short symphysis is formed by the junction of the pubic epiphyses; and the upper molars are generally small, and Insectivorous Otter. triangular, with three cusps arranged in a V. The first family, Potamogalidae, is represented by the otter-like Potamogale velox of the rivers of West Africa (fig. 8), distinguished from all other members of the order by the absence of clavicles. The tibia and fibula are united interiorly, the skull has a ring-like tympanic, no zygomatic arch, and the upper molars are of the tuberculo-sectorial type, with broader crowns than in the following families. The dental formula is i. 3/3, c. 1/1, p. 3/3, m. 3/3, total 40. This animal inhabits the banks of streams in west equatorial Africa, and its whole structure indicates an aquatic life. It is nearly 2 ft. in length, the tail measuring about half. The long cylindrical body is continued uninterruptedly into the thick laterally compressed tail, the legs are very short, and the toes are not webbed, progression through the water depending wholly on the action of the powerful tail, while the limbs are folded inwards and backwards. The muzzle is broad and flat, and the nostrils are protected by valves. The fur is dark brown above, the extremities of the hairs on the back being of a metallic violet hue by reflected light, beneath whitish.

Fig. 9.—Skull of the Tenrec (Centeles ecaudatus), somewhat reduced.

In the remaining groups the upper molars form narrow V’s of the true tritubercular type. The family, Centetidae, represented by the tenrec and a number of allied animals from Madagascar, is specially characterized by the ring-like tympanic, and the absence of a zygomatic arch and of any constriction of the skull Tenrec. behind the orbits, and the presence of teats on the breast as well as the abdomen. In the more typical members of the family the tibia and fibula are separate, and, as in hedgehogs, spines are mingled with the fur. The true or great tenrec (Centetes ecaudatus), alone representing the typical genus, has the dental formula i. 3 or 2/3, c. 1/1, p. 3/3, m. 3 or 4/3 or 4, total 38, 40, 42 or 44. The fourth lower molar, when developed, does not appear till late in life. Of the long and sharp canines, the tips of the lower pair are received into pits in the upper jaw (fig. 9). The creature grows to a length of aoout a foot. The young have strong white spines arranged in longitudinal lines along the back, but these are lost in the adult which has only a crest of long rigid hairs on the nape of the neck. The lesser tenrecs, Hemicentetes semispinosus and H. nigriceps, are distinguished by the persistence of the third upper incisor and the form of the skull. The two species are much smaller than the great tenrec, and spines are retained in the adult on the body. The hedgehog-tenrec, Ericulus setosus, has the whole upper surface, and even the short tail, densely covered with close-set spines. The facial bones are much shorter than in the preceding genera, and the first upper incisors are elongated; while there are only two pairs of incisors in each jaw. Judging from the slight development of the cutaneous muscles compared with those of the hedgehog, it would seem that these creatures cannot roll themselves completely into balls in hedgehog-fashion. A second species of this genus, Ericulus (Echinops) telfairi, has two, in place of three, pairs of molars, thus reducing the total number of teeth to 32. Moreover, the zygomatic arches of the skull are reduced to mere threads. Here should perhaps be placed Geogale aurita, a small long-tailed Malagasy insectivore, with 34 teeth, and no spines; the tibia and fibula being separate. It has been classed in the Potamogalidae, but from its habitat such a reference is improbable. The absence of spines may entitle it to separation from the Centetinae, so that it should perhaps be regarded as representing a subfamily, Geogalinae, by itself.

Fig. 10.—Skull of the Lesser Tenrec (Hemicentetes spinosus).

The absence of spines coupled with the union of the tibia and fibula form the leading characteristics of the subfamily Oryzorictinae, typified by the rice-tenrecs Oryzorictes, of which there are several species. These creatures, which excavate burrows in the rice-fields of Madagascar, are somewhat mole-like in appearance, but have tails of considerable length. In the typical O. hova the fore-feet are five-toed, but in O. tetradactylus the number of front digits is reduced to four. The long-tailed tenrecs (Microgale) are represented by fully half-a-dozen species with tails of great length; that appendage in the typical M. longicaudata being more than double the length of the head and body, and containing no fewer than forty-seven vertebrae. The teeth are generally similar to those of Centetes, but are not spaced in front; their number being i. 3/3, c. 1/1, p. 3/3, m., 3/3, total 40, or the same as in Oryzorictes. Finally, Limnogale mergulus, a creature about the size of a black rat, has webbed toes and a laterally compressed tail, evidently adapted for swimming. See Tenrec.

Fig. 11.—Solenodon cubanus.

All the foregoing are natives of Madagascar. It has been suggested however, that two remarkable West Indian insectivores, namely Solendon cubanus of Cuba (fig. 11) and S. paradoxus of Hayti, should be regarded as representing merely a subfamily of Centetidae. It is true that the main features Solenodon. distinguishing these strange creatures from the Malagasy representatives of that family are the constriction of the skull behind the orbits, the descent of the testes into the perineum, and the post-inguinal position of the teats, and that none of these are of very great importance. But the geographical positions of the two groups are so widely sundered that it seems preferable to await further evidence before definitely assigning the two to a single family; and the family Solenodontidae may accordingly be retained for the West Indian animals. Solenodons, which look like huge long-nosed, parti-coloured rats, have the tibia and fibula separate, and the same dental formula as Microgale. Each of the two species (which differ in colour and the quality of the fur) has a long cylindrical snout, an elongated naked tail, feet formed for running, and the body clothed with long, coarse fur. The position of the teats on the buttocks is unique among Insectivora. The first upper incisors are much enlarged, and like the other incisors, canines and premolars, closely resemble the corresponding teeth of Myogale; the second lower incisors are much larger than the upper ones, and hollowed out on the inner side.

Fig. 12.—A Golden Mole (Chrysochloris obtusirostris) reduced.

The last family, Chrysochloridae, is represented by the golden moles of South and East Africa, which differ from the Centetidae and Solenodontidae by the development of a bulla to the tympanic, and the presence of a zygomatic arch to the skull; the tibia and fibula being separate, and the symphysis {{{2}}} of the pelvis formed merely by ligament. The skull is not constricted across the orbits. The teats, which are placed both on the breast and in the groin, are situated in shallow depressions. The ears are buried in the fur, and the eyes concealed beneath the skin; the feet are four-toed and provided with powerful claws for burrowing in the fashion of the mole, but it is interesting to note that the skeleton is modified for the same purpose in a manner quite different from that obtaining in the latter animal. These animals derive their name from the metallic iridescence of the fur of most of the species. In the more typical species the dental formula is the same as in Microgale, that is to say, there are 40 teeth. In other species, which it has been proposed to separate as Amblysomus, there are, however, only 36 teeth, owing to the absence of the last pair of molars. The group is evidently nearly related to the Centetidae—most nearly perhaps to the Oryzorictinae.

Fossil Insectivora.

Some years ago Dr F. Ameghino, of Buenos Aires, described from the Tertiary formation of Santa Cruz, in Patagonia, the remains of an insectivore under the name of Necrolestes. The occurrence of a member of the Insectivora in these beds is remarkable, since this group is represented at the present day in South America only by a shrew or two which have wandered from the north. Dr Ameghino expressed his belief that the extinct Patagonian insectivore was nearly related to the golden moles, and although this opinion appears to have been withdrawn, Professor W. B. Scott states that he is convinced of the close affinity existing between Necrolestes and Chrysochloris. Although this view may not be accepted, it must be remembered that it represents the opinion of a palaeontologist who has had better opportunities than most of his fellow-workers of forming a trustworthy judgment. So convinced is Dr Scott of the closeness of the relationship between Necrolestes and the golden moles that he regards it as rendering probable the former existence of a direct land-connexion between Africa and South America. There is no reason, he says, to suppose that the track of migration could have been by way of Europe and North America, for no trace of the group has been found anywhere north of the equator. This supposed connexion between Africa and South America in Tertiary times has often been suggested, and is supported by many independent lines of evidence; and the presumed affinity between the two mammals here referred to adds to the weight of such evidence.

The discovery in the Oligocene Tertiary deposits of Dakota of the remains of a species of hedgehog is a fact of great interest, for the hedgehog-tribe (Erinaceidae) is at the present day an exclusively Old World group. The discovery of the fossil American species, which has been made the type of a new genus under the name of Protherix, serves to strengthen the view that the northern countries of the Western and Eastern hemispheres form a single zoological region; and that formerly there was comparatively free communication between them in the neighbourhood of Bering Sea, under climatic conditions which permitted of temperate forms passing from one continent to the other. As might have been expected, remains of hedgehog-like mammals have been obtained in the Tertiary deposits of Europe. Among these, Palaeoerinaceus, from the Upper Oligocene of France, seems scarcely separable from the existing genus. Necrogymnurus (Neurogymnurus) from the Lower Oligocene, of the same country, appears to be allied to Hylomys, which is itself the most generalised of the family, so that the extinct genus, of which Caluxotherium is a synonym, may represent the ancestral type of the Erinaceidae. The genus Galerix, or Lanthanotherium, of the Oligocene, which has the typical series of 44 teeth, a bony ring round the orbit, and conjoint tibia and fibula, has been regarded as representing the Tupaiidae and Macroscelididae, but is more probably referable to the Erinaceidae, being apparently akin to Gymnura. The moles are represented in the French Oligocene by Amphidozotherium and in the Miocene by Talpa, while in the North American early Tertiary we have the primitive Talpavus. Shrews are also known from the Lower Oligocene upwards both in the eastern and western hemispheres. Of the Lower Eocene Adapisorex, with the typical 22 lower teeth, Adapisoriculus and Orthaspidotherium, all from France, the affinities are quite uncertain. The American Oligocene Leptictis, with i. 2, c. 1, p. 4, m. 3 in the upper jaw, and Ictops, with i. 3/2, c. 1/1, p. 4/4, m. 3/3, may be insectivorous mammals, with affinities to the creodont Carnivora. It is, indeed, probable that not only is there a relationship between the Creodonta and the Insectivora, but also one between the latter and the Marsupialia, so that the marked similarity between the cheek-teeth of the insectivorous Chrysochloris and the Marsupial Notoryctes may be due to genetic relationship. That the bats and the flying-lemur are descendants of the Insectivora cannot be doubted.

Bibliography.—G. E. Dobson, “Monograph of the Insectivora” (London, 1883–1890); W. Leche, “Zur Morphologie des Zahnsystems der Insectivoren,” Anatom. Anzeiger (xiii. 1 and 514, 1897); C. J. Forsyth-Major, “Diagnoses of New Mammals from Madagascar.” Ann. Mag. Nat. Hist. ser. 6. vol. xviii. pp. 318 and 461 (1896): A. A. Mearns, “Descriptions of New Mammals from the Philippine Islands,” Proc. U.S. Museum (xxviii. 425, 1905).  (R. L.*)