Tribe IV. POEPRAGA.
The present tribe includes the most strictly vegetable feeders; all the species have a com plex sacculated stomach, and a long simple ccecum.
Genus HYPSIPHYM NUS. Potoroos.
Guided by the modifications of the teeth we pass from the Koala to the Potoroos and Kangaroos—animals of widely different general form. The Potoroos, however, present absolutely the same dentition as does the Koala, some slight modifications in the form of certain teeth excepted. The premolars in their longi tudinal extent, compressed form, and cutting edge, would chiefly distinguish the dentition of the Potoroo ; but the Koala evidently offers the transitional structure between the I'halangers and Potoroos in the condition of these teeth, of which one only is retained on each side of each jaw in the Potoroos as in the Koala.
The dental formula of Hypsiprymnus, the generic name of the Potoroos, is The two anterior incisors are longer and more curved, the lateral incisors relatively smaller than in the Koala. The pulps of the anterior incisors are persistent. The canines are larger than in the Koala ; they always project from the line of the intermaxillary suture ; and, while the fang is lodged in the maxillary bone, the crown projects almost wholly from the in termaxillary. In the large Hypsiprymnas ursi nus the canines are relatively smaller than in the other Potoroos, a structure which indicates the transition from the Potoroo to the Kangaroo genus. In the skeleton of this species in the Leyden Museum, the canines have a longitu dinal groove on die outer side.
The characteristic form of the trenchant pre molar has just been alluded to : its maximum of development is attained in the arboreal I'o toroos of New Guinea ( Hypsiprymnus ursinus and Ilyps. dorcocephalus), in the)atter of which its antero-posterior extent nearly equals that of the three succeeding molar teeth. In all the Potoroos the trenchant spurious molar is sculp tured, especially on the outer side, and in young teeth, by many small vertical grooves. The true molars each present four three-sided pyramidal cusps ; but the internal angles of the two opposite cusps are continued into each other across the tooth, forming two angular or concave transverse ridges. In the old animal these cusps and ridges disappear, and the grind ing surface is worn quite flat, as in fig,. 91,
which represents the dentition of the original Potoroo, described in White's Voyage.
Genus MACROP US. Kangaroos.
In the genus 3facropus (fig. 92) the normal condition of the permanent teeth may be ex pressed as follows:— 0-0 3— 3 Incisors, —; canines, • premo 1 — 0 -- 0 1 — I 4 — 4 lays, —; molars, = 28.
1 — 1 4 — 4 The main difference, as compared with Hyp siprymnus, lies in the absence of the upper canines as functional teeth ; the germs, how ever, of these teeth are always to be found in the young mammary foetus of the Macropus major, and I have seen them present, but of very small size, and concealed by the gum, in the adults of some small species of kangaroos, as Macropus rufiventer, Ogilby, and Muer. psilopus, Gould. This, however, is a rare ex ception ; while the constant presence and con spicuous size of the canines will always serve to distinguish the Potoroo from the Kangaroo. But there are also other differences in the form and proportions of certain teeth. The upper incisors of the Macropi have their cutting margins in the same line, the anterior ones not being produced beyond thatline, as in the Hyp siprymni : the third or external incisor is also broader in the kangaroos, and is grooved and complicated by one or two folds of the enamel, continued from the outer side of the tooth ob liquely forward and inward. Iu most species the anterior fold is represented by a simple groove : the relative size of the outer incisor, the extent and position of the posterior fold of enamel, and consequently the proportions of the pact of the tooth in front or behind it, vary more or less in every species of Macropus there are two folds of enamel near the anterior part of the tooth in Macr. major, and the pos terior portion is of the greatest extent, and the entire crown of the tooth is relatively broadest in this species. The middle incisor is here also complicated by a posterior notch and an external groove. These modifications of the external incisors of the kangaroos were first no ticed by Mr. Jourdan, and subgeneric distinc tions, with names often sufficiently unmeaning, if not absurd,* have been subsequently based upon them -, but such dental characters possess neither sufficient constancy nor physiological importance to justify such an application.