there are no external ear-conchs; and the dentition includes four pairs
of upper, and three of lower, incisors, and distinctly tritubercular
cheek-teeth. The small pouch, supported by the usual epipubic
bones, opens backwards. In correlation with its burrowing habits,
some of the vertebrae of the neck and of the loins are respectively
welded together. The eyes have degenerated to a greater extent than
those of any other burrowing mammal, the retina being reduced to a
mass of simple cells, and the cornea and sclerotic (“white”) to a pear-shaped
fibrous capsule enclosing a ball of pigment. The reason for
this extreme degeneration is probably to be found in the sandy nature
of the soil in which the creature burrows, a substance which would
evidently irritate and inflame any functional remnant of an eye.
The portion of the lachrymal duct communicating with the cavity
of the nose has, on the other hand, been abnormally developed,
apparently for the purpose of cleansing that chamber from particles
of sand which may obtain an entrance while the animal is burrowing.
(See Marsupial Mole.)
| ||
| Fig. 7.—The Pig-footed Bandicoot (Choeropus castanotis). |
2. Paucituberculates.—The second sub-order of marsupials, the Paucituberculata, is exclusively South American, and typically represented by the family Epanorthidae, the majority of the members of which are extinct, their remains being found in the probably Miocene Santa Cruz beds of Patagonia, although one existing genus (Caenolestes) survives in Ecuador and Colombia. One of the two living species was, indeed, described so long ago as the year 1863, under the preoccupied name of Hyracodon, but attracted little or no attention, as its affinities were not fully recognized. Externally Caenolestes has a shrew-like appearance. The elongated skull (fig. 8) has four pairs of upper incisors and long upper canines, while in the lower jaw there is a single pair of procumbent incisors, followed by several small teeth representing the canine and earlier premolars. The three pairs of molars in each jaw are, like the last premolar, quadritubercular oblong teeth. The five-toed feet are of normal structure, and the rat-like tail is prehensile towards the tip. The female has a small pouch. The extinct members of the family are represented by the genera Epanorthus, Acdestis, Garzonia, &c. In a second family—Abderitidae—also from the Patagonian Miocene, the penultimate premolar is developed into an enormous tooth, with a tall, secant and grooved crown, somewhat after the fashion of the enlarged premolar of Plagiaulax. From the structure of the skull, it is thought probable that Abderites had an elongated snout, like that of many Insectivora. As a sub-order, the Paucituberculata are characterized by the presence of four pairs of upper and three of lower incisor teeth; the enlargement and forward inclination of the first pair of lower incisors, and the presence of four or five sharp cusps on the cheek-teeth, coupled with the absence of “syndactylism” in the hind limbs.
| ||
| Fig. 8.—Skull of Caenolestes obscurus. |
3. Diprotodonts.—The third and last sub-order of marsupials is the Diprotodontia, which is exclusively Australasian and includes the wombats, koala, cuscuses, kangaroos and their relatives. There are never more than three pairs of upper and one of lower incisors, of which the middle upper and the single lower pair are large and chisel-like (fig. 9); the canines are small or absent; the cheek-teeth have bluntly tuberculate or transversely-ridged crowns in most cases; and the hind-feet are syndactylous. With one exception, the intestine has a caecum, and the pouch is large and opens forwards. It should be added that Professor Elliot Smith has pointed out a certain peculiarity in its commissures whereby the brain of the diprotodonts differs markedly from that of the polyprotodonts and approximates to the placental type. Dr Einar Lönnberg has also recorded certain adaptive peculiarities in the stomach. Most of the species, particularly the specialized types, are more or less completely herbivorous.
| ||
|
Fig. 9.—Front view of Skull of the Koala (Phascolarctus cinereus) |
The first family, Phascolomyidae, is typified by the wombats; but according to the view adopted by Mr H. Winge, and endorsed by Professor Max Weber, is also taken to include the koala. In this wider sense the family may be characterized as follows. The tympanic process of the alisphenoid bone of the skull is short, not covering the cavity of the tympanum, nor reaching the paroccipital process. The tail is rudimentary, the first hind-toe opposable, the first pair of upper incisors very large, but the second and third either absent or small and placed partially behind the larger pair; and only five pairs of cheek-teeth in each jaw. The stomach has a cardiac gland, and the number of teats is two.
In the wombats (Phascolomys) the dentition is i. 11, c. 00, p. + m. 55, total 24; all the teeth growing from persistent pulps, and the incisors large and chisel-like, with enamel only on the front surface. The cheek-teeth strongly curved, forming from the base to the summit about a quarter of a circle, the concavity being directed outwards in the upper and inwards in the lower teeth. The first of the series (which appears to have no predecessor) single-lobed; the other four composed of two lobes, each subtriangular in section. Limbs equal, stout and short. Fore-feet with five distinct toes, each furnished with a long, strong and slightly curved nail, the first and fifth considerably shorter than the other three. Hind-feet with a very short nailless first toe, the second, third and fourth toes partially united by integument, of nearly equal length, the fifth distinct and rather shorter; all four with long and curved nails. In the skeleton the second and third toes are distinctly more slender than the fourth, showing a tendency towards the character so marked in the following families. Tail rudimentary. Caecum very short and wide, with a vermiform appendage (see Wombat).
In addition to remains referable to the existing genus, the Pleistocene deposits of Australia have yielded evidence of an extinct giant wombat constituting the genus Phascolonus (Sceparnodon).
The koala, or “native bear” (Phascolarctus cinereus), which differs widely from the wombats in its arboreal habits, is less specialized as regards its dentition, of which the formula is i. 31, c. 10, p. + m. 55, total 30. Upper incisors crowded together, cylindroidal, the first much larger than the others, with a bevelled cutting edge (fig. 9). Canine very small; a considerable interval between it and the first premolar, which is as long from before backwards but not so broad as the molars, and has a cutting edge, with a smaller parallel inner ridge. The molar-like teeth slightly diminishing in size from the
