MAIISUPIALIA, 



265 



gressively increasing in length as they are 

 placed forward. An interspace occurs between 

 these and the canine, which is long, slender, 

 sharp-pointed, and recurved. The first incisor 

 is longer than the two behind, but is much 

 shorter than the canine. In the lower jaw the 

 true molars are preceded by two functional 

 false ones, similar in size and shape to the 

 three above the anterior false molar, and the 

 canine are represented by minute rudimental 

 simple teeth; the single incisor is long and 

 procumbent as in the other Petaurists. 



With these differences of dentition approach- 

 ing more or less to one or other of the modifi- 

 cations of the dentition in the group of Phalan- 

 gers, the Petaurists may nevertheless be readily 

 discriminated from those Phalangers which 

 they most resemble ; for example, the PeUturus 

 Taguunoides may be distinguished from the 

 Phalangista Cookii by the greater relative 

 length in the latter of the nasal and maxillary 

 portion of the skull ; while in most of the other 

 species of Petaurus, the facial part of the skull 

 is relatively shorter than in the Pet. Tagua- 

 noides. 



Genus PHASCOLARCTUS. 



The absence of anomalous or functionless 

 premolars and of inferior canines appears to be 

 constant in the only known species of this 

 genus. The dental formula in three examples 

 of this species (Phaac.J'uscus, Desm.) is 



o o J J 



Incisors ; canines ; premo- 



lars 



11 



11 



molars 



00 



44 



: = 30. (Fig. 95.) 

 11 ' 44 



The true molars are larger in proportion than 

 in the Phalangers ; each is beset with four 

 three-sided pyramids, the cusps of which wear 

 down in age, the outer series in the upper teeth 

 being the first to give way ; those of the lower 

 jaw are narrower than those of the upper. The 

 spurious molars are compressed and terminate 

 in a cutting edge ; in those of the upper jaw 

 there is a small parallel ridge along the inner 

 side of the base. The canines slightly exceed 

 in size the posterior incisors; they terminate in 

 an oblique cutting edge rather than a point ; 

 their fang is closed at the extremity : they are 

 situated as in the Phalangers close to the inter- 

 maxillary suture. The lateral incisors of the 

 upper jaw are small and obtuse ; the two ante- 

 rior or middle incisors are twice as long, broad, 

 and thick as the posterior incisors ; they are 

 conical, slightly curved, sub-compressed, be- 

 velled off obliquely to an anterior cutting edge, 

 but differing essentially from the denies scal- 

 ;;rariiofthe Rodentia in being closed at the 

 extremity of the fang. The two incisors of the 

 lower jaw resemble those of the upper, but are 

 larger and more compressed ; they are also 

 formed by a temporary pulp, and its absorption 

 is accompanied by a closure of the aperture of 

 the pulp cavity, as in the upper incisors. The 

 Koala, therefore, in regard to the number, kind, 

 and conformation of its teeth, closely resembles 

 the Phalangers, with which it also agrees in its 

 long coecum and the general conformation of 

 its digestive organs. It has also the extremities 



similarly organised for prehension; each is ter- 

 minated by fivedigits; the hind feet are provided 

 with a large thumb, and have the two contiguous 

 digits enveloped in the same tegumentary fold ; 

 the anterior digits are divided into two groups ; 

 the thumb and index being opposed to the 

 other three fingers. We have already noticed 

 a structure approaching to this in some of the 

 small Phalangers. The Koala, however, differs 

 from the Phalangers and Petaurists in the ex- 

 treme shortness of Us tail, and in its more com- 

 pact and heavy general form. It is only known 

 to feed on the buds and leaves of the trees in 

 which it habitually resides. 



Tribe IV. POEPHAGA. 

 The present tribe includes the most strictly 

 vegetable feeders; all the species have a com- 

 plex sacculated stomach, and a long simple 

 ccccum. 



Genus HYPSIPRYMNUS. Potoroos. 

 Guided by the modifications of the teeth 

 pass from the Koala to the Potoroos 



we 



and Kangaroos animals of widely different 

 general form. The Potoroos, however, present 



Fig. 91. 



Hypsiprymnus murinug. 



absolutely the same dentition as does the Koafa, 

 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 Phalangers 

 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 



O O -4 | 



Incisors -ZI_ ; canines . : ; premo- 

 11 00 



lars -ZL ; molars . 4 4 : = 30. 

 11 44 



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 hone, 

 the crown projects almost wholly from the in- 

 termaxillary. In the large Hypsipn/mnm 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 



