M. Fr. Cuvier has proposed a binary divi sion of the genus Afacropus, as here defined, founded on the absence of permanent spurious molars, and a supposed difference in the mode of succession of the true molars in certain spe cies of Kangaroo, combined with modifications of the muzzle or upper lip, and of the tail.
The dental formula which I have assigned to the genus Macropus is restricted in its ap plication by that naturalist to some small spe cies of Kangaroo, grouped together under the term Halmaturus, originally applied by Illiger to the Kangaroos generally.$ The rest of the Kangaroos, under the generic term Illacro pus, are characterised by the following dental molars, 4 — 4 = 24.
2 4 — 4 The truth, however, is, that both the Halmaturi and Macropi of M. Fr. Cuvier have their teeth developed in precisely the same num ber and manner : they only differ in the length of time during which certain of these teeth are retained.; In the great Kangaroo, for exam ple, the permanent premolar which suceeeds the corres ponding deciduous one in the vertical direction, is pushed out of place and shed by the time the last true molar has cut the gum : the succeeding true molar is soon afterwards extruded ; and I have seen a skull of an old Macropus major in the Museum at Leyden, in which the grinders were reduced to two on each side of each jaw by this yielding of the anterior ones to the vis a tergo of their successors.
The general form of the body in the Ma cropodidee is that of an elongated cone, the broad and stout haunches forming the base, and the produced tapering muzzle the apex. The proportions of the body are, however, re duced by so elegant a gradation that they are justly considered as among the most picturesque of quadrupeds. The hinder extremities are al ways longer and stronger than the fore ones, but in various proportions ; the difference being least in the arboreal Potoroos, and in that section of the genus represented by the Hypsiprymnus myosurus of Van Dieman's Land. The tail is very long in all the species, but is strongest in the great kangaroos, which make use of it as a kind of crutch or fifth extremity in their slower modes of progression. In the Potoroos the tail is more slender, and in these and some of the smaller species of kangaroo it is bent be neath the body when the animal reposes.
Tribe V. RHIZOP HA G A.
In this tribe, the stomach is simple in out ward form, but complicated within by a large cardiac gland ; and the ccecum, which is short and wide, with a vermiform appendage.
Genus PHASCOLOMYS, ( fig. 93.) In its heavy shapeless figure, large trunk, and short equably developed legs, the 'Wom bat offers as great a contrast to the Kangaroos as does the Koala, which it. most nearly resembles in its general outward form and want of tail.
But in the more important characters afforded by the teeth and intestinal anal, the Wombat differs more from the Koala than the latter does from either the Phalangers or Kangaroos.
The dental system presents the extreme de gree of that degradation of the teeth, interme diate between the front incisors and true molars, which we have been tracing from the Opossum to the Kangaroos : not only have the function less premolars and canines now totally disap peared, but also the posterior incisors of the upper jaw, which we have seen in the Potoroos to exhibit a feeble degree of development as compared with the anterior pair; these in fact are alone retained in the dentition of the present group, the representative of which possesses the fewest teeth of any Marsupial animal. The dental formula of the Wombat is thus reduced both in number and kind to that of the true Rodentia.* The incisors moreover are true denies scalpra rii, with persistent pulps, but are inferior, espe cially in the lower jaw, in their relative length and curvature to those of the placental Glires; they presenta subtriedral figure, and are traversed by a shallow groove on their mesial surfaces.
The spurious molars present no trace of that compressed structure which characterizes them in the Koala and Kangaroos, but have a wide oval transverse section ; those of the upper jaw being transversed on the inner side with a slight longitudinal groove. The true molars are double the size of the premolars : the su perior ones are also traversed by an internal longitudinal groove, but this is so deep and wide that it divides the whole tooth into two prismatic portions, with one of the angles directed inwards. The inferior molars are in like manner divided into two triedral portions, but the intervening groove is here external, and one of the facets of each prism is turned inwards. All the grinders are curved, and describe about a quarter of a circle : in the upper jaw the con cavity of the curve is directed outwards ; in the lower jaw, inwards. The false and true molars, like the incisors, have per sistent pulps, and are consequently devoid of true fangs, in which respect the Wom bat differs from all other Marsupials, and resembles the extinct Toxodon, the denti n.,,,,,c B,.uta andherbivorous 12ndontin I may add that the Wombat deviates from the other Marsupials in the number of its ribs; as these are very constant in the rest of the order, the difference in the Wombat, which has 15 pairs, instead of 13 or 12, is the more deserving of notice. The Koala, like the Phalangers and Kangaroos, has 13 pairs of ribs ; but this class of characters will form the subject of the fol lowing section.