deposits of various parts of Russia, Rumania, and Siberia, and
others from the Lower Pliocene of northern India; the molar teeth
of these latter presenting the additional column referred to above
as distinguishing those of the llamas from those of modern camels.
In addition to these Dr M. Schlosser has described remains of a large
camel-like animal from China, with apparently generalized affinities,
for which the name of Paracamelus is proposed. Mme Pavlow,
of Moscow, has brought to notice a fossil camel-skull of great
interest, which was collected in the district Alexandrie, of the government
of Kherson, Russia. Unfortunately, the precise age of the
formation from which it was obtained is unknown, but it is
considered probable that it dates from the later Tertiary. Although
it has the deciduous dentition, Mme Pavlow considers herself
justified in referring the Kherson skull to the genus Procamelus
previously known only from the Lower Pliocene or Upper Miocene
strata of North America, and differing from modern camels, among
other features, by the retention of a fuller, series of premolar teeth.
Part of the cannon-bone of a camel from another district in Russia
is provisionally assigned to the same species. Possibly this Russian
camel (Procamelus khersonensis), as it is called, may form the
connecting link between the typical Procamelus of North America
and the fossil camel (Camelus sivalensis) of the Siwalik Hills of
India. Be this as it may, the identification of a North American
type of camel from the Tertiary strata of eastern Europe forms
another connecting link between the extinct faunas of the northern
half of the Old World and North America, and thus tends to show
that the claim of America to be the exclusive birthplace of many
Old World types may have to be reconsidered.
Remains of camels (C. thomasi) have also been found in the Pleistocene strata of Oran and Ouen Seguen, in Algeria; and certain remains frorn the Isle of Samos have been assigned to the same genus, although the reference requires confirmation. The Algerian Pleistocene camel was doubtless the direct ancestor of the living African species, which it serves to connect with the extinct C. sivalensis.
In North America, apart from certain still older and more primitive mammals, with teeth of the tubercular type, the earliest known form which can definitely be included in the camel-series is Protylopus, of the Uinta or Upper Eocene. In this creature, which was not larger than a European hare, there was the full number of 44 teeth, which formed a regular series, without any long gaps, and with the canines but little taller than the incisors, while the hinder cheek-teeth, although of the crescentic type, were low-crowned. In both jaws the anterior front-teeth were of a cutting and compressed type. Unfortunately, the skull is incomplete, and the rest of the skeleton very imperfectly known; but sufficient of the former remains to show that the socket of the eye was open behind, and of the latter to indicate that in the hind-foot, at any rate, the upper bones of the two functional toes had not coalesced into a cannon-bone. The lateral hind-toes (that is to say the second and fifth of the typical series) had, however, become rudimentary; although it is probable that the corresponding digits of the forelimb were functional, so that this foot was four-toed. In old individuals the bones of the forearm (radius and ulna) became welded together about half-way down, although they remained free above. On the other hand it appears that the smaller bone of the leg (fibula) was welded to the larger one (tibia), and that its upper portion had disappeared. Nothing is known of the neck vertebrae. It is, of course, evident that there must have been an earlier form in which all the feet were four-toed, and the bones of the forearm and lower part of the leg separate.
A stage higher in the series, viz. in the Oligocene, we meet with Pöebrotherium, in which a distinct increase in bodily size is noticeable, as also in the relative length of the two bones which unite in the higher types to form the cannon-bone. Moreover, the crowns of the hinder cheek-teeth are taller, and more distinctly crescentic, both feet are two-toed, the ulna and radius are fused, and the fibula is represented only by its lower part. In the vertebrae of the neck the distinctive cameloid characters had already made their appearance. On the other hand, the skull was short and rabbit-like, showing none of the characteristic features of modern camels.
In the Lower Miocene occurs Protomeryx or Gomphotherium, in which there is a considerable increase in the matter of bodily size, the two metacarpal and metatarsal bones (or those which unite in the latter forms to constitute the cannon-bones) being double the length of the corresponding elements in Protylopus. These bones, although separate, have their adjacent surfaces more closely applied than is the case in the latter; while in this and the earlier genera the terminal toe-bones indicate that the foot was of the normal hoofed type. In the skull the socket of the eye is surrounded by bone; while the dentition begins to approximate to the camel type—notably by the circumstance that the lower canine is either separated by a gap from the outermost incisors, or that its crown assumes a backwardly curved shape. In Protolabis of the Middle Miocene, while no cannon-bone is formed, the first and second pairs of incisor teeth are retained, and the limbs and feet are short and disproportionately small. In the Upper Miocene we come to a distinct type—Procamelus—which is entitled to be regarded as a camel, and approximates in size to a small llama. Here the metacarpals and metatarsals have partially united to form cannon-bones, the skull has assumed the elongated form characteristic of modern camels, with the loss of the first and second pairs of upper incisors, and the development of gaps in front of and behind each of the next three teeth, that is to say, the third incisor, the canine and the first cheek-tooth. The approximately contemporaneous Pliauchenia makes another step by the loss of the second lower cheek-tooth. Both these genera have the toe-bones of the irregular nodular form distinctive of modern camels, so that we may safely infer that the feet themselves had assumed the cushion-type.
In one species of Procamelus the metacarpals and rnetatarsals coalesced into canon-bones late in life; but when we come to the Pleistocene Camelops such union took place at an early stage of existence, and was thoroughly complete. In the living members of the group it occurs before birth. The species of Camelops were probably fully as large as llamas, and some, at any rate, resembled these animals as regards the number of teeth, the incisors being reduced to one upper and three lower pairs, and the cheek-teeth to four or five in the upper and four in the lower jaw; the total number of teeth thus being 28 or 30 in place of the 44 of Pöebrotherium. The sole difference between Camelops and Llama seems to consist in certain structural details of the lower cheek-teeth. An allied extinct genus (Eschatius) is also distinguished by certain features in the dentition.
Apart from Procamelus the foregoing genera are exclusively North American. A lower jaw from the Pleistocene deposits of that continent has, however, been referred to the Old World Camelus.
In addition to the above there is an extraordinary North American Miocene giraffe-necked camel (Alticamelus), a creature of the size of a giraffe, with similarly elongated neck and limbs, and evidently adapted for browsing on trees. The feet and number of teeth were generally similar to those of Procamelus. Unlike the giraffe, the length of the limbs is due to the elongation of their upper segments, and that of the neck to the lengthening of only the hinder vertebrae.
In caverns and superficial deposits of South America occur remains of extinct species more or less closely related to modern llamas; but previous to the Upper Pliocene the group is unknown in South America, which it reached from the north.
All the foregoing genera are included in the sub-family Camelidae. Parallel to this is, however, the North American family Leptomerychidae (Hypertragulidae), as represented by Leptomeryx, Camelomeryx and Leptoreodon, which presents remarkable resemblances, especially in the type genus, to the Tragulina (see Chevrotain); camel-like features being, however, apparent in the two genera last mentioned. Generalized features are also displayed by the Oligocene Hypisodus, which in its short skull and large orbits presents a curious approximation to the African dik-dik antelopes of the genus Madoqua (see Antelope). Again, the remarkable horned North American Oligocene genus Protoceras, while displaying resemblances to Leptomeryx and Leptoreodon, presents also points of similarity to the Tragulina and Pecora (q.v.).
The North American genus Oreodon typifies a second family included by Professor W. B. Scott in the Tylopoda and generally known as the Oreodontidae. As Oreodon is, however, antedated by Merycoidodon, the latter name is properly entitled to stand, in which case the family should be called Agriochoeridae. It is not easy to point out the characters in which the family approximates to the Camelidae, and only its general characteristics can be indicated. The family ranges in North America from the Upper Eocene to the Lower Miocene, but Oreodon (or Merycoidodon), which is typified by an animal of the size of a sheep, is Oligocene. In the Oreodontinae or typical section of the family, which includes several genera nearly allied to Oreodon, the skull is shorter and higher than in the camels, with a swollen brain-case, a preorbital gland-pit, the condyle of the lower jaw transversely elongated, the tympanic bulla hollow, and the orbit surrounded by bone. The dentition comprises the typical 44 teeth, of which the molars are short-crowned, with four crescentic cusps on those of the upper jaw (selenodont type). The most characteristic dental feature is, however, the assumption of the form and function of a canine by the first lower premolar; the lower canine being incisor-like. The tail is very long; and the feet have five functional toes, with complete but short metacarpals or metatarsals. In the Miocene Agriochoerus, which typifies a second sub-family (Agriochoerinae), there is no gland-pit in the skull, of which the orbit is open behind; while the upper incisors are wanting in the adult and the terminal toe-bones are claw-like rather than of the hoofed type. The molars are less completely selenodont than in the type genus. It is noteworthy that a molar from the Tertiary of India has been referred to Agriochoerus, a determination which if correct probably indicates the occurrence of Oreodonts in the unknown Tertiary deposits of Central Asia. It may be added that in the Oreodontidae the vertebral artery pierces the transverse processes of the cervical vertebrae in the normal manner.
The earliest representatives of the Tylopoda according to Professor Scott is the Middle Eocene genus Homacodon, typifying the family Homacodontidae, which is regarded as the common ancestor of both
