292 



ANATOMY OF VERTEBRATES. 



which work against a similar but straight procumbent pair of 

 incisive tusks below ; thus presenting a transitional feature 

 between the Kangaroos and the Rodent form of Marsupial called 

 Wombat (Phascolomys). 1 In this genus, the dental system pre- 

 sents the extreme degree of that degradation of the teeth, inter- 

 mediate between the front incisors and true molars, which has been 

 traced from the Opossum to the Kangaroos ; not only have the 

 functionless premolars and canines now totally disappeared, but 

 also the posterior incisors of the upper jaw, which we have seen 

 in the Koala and Potoroo to exhibit a feeble degree of develop- 

 ment as compared with the anterior pair ; these, in fact, are alone 

 retained in the dentition of Phascolomys. The dental formula of 

 the Wombat is thus reduced, both in number and kind, to that 

 of Rodentia, viz. — 



.2 o 1.1 4.4 



V' r ;/> Li ; ™ ^ = 24, fig. 232. 



The incisors, /, moreover, are ' dentes scalprarii,' but are in- 

 ferior, especially in the lower jaw, in their relative length and cur- 

 232 vature to those of the 



placental Glires ; they 

 present a subtriedral 

 figure, and are tra- 

 versed by a shallow 

 groove on their mesial 

 surface. The premolars, 

 p, 3, present no trace 

 of that compressed 

 structure which cha- 

 racterises them in the 

 Koala and Kangaroos, 

 but have a wide oval 

 transverse section ; those of the upper jaw being traversed, on 

 the inner side, by a longitudinal groove. The true molars, 

 m 1-4, are double the size of the premolars ; the superior 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 inward. The 

 inferior molars are in like manner divided into two triedral 

 portions ; but the intervening groove is external, and one of the 

 facets of each prism is turned inward. All the grinders are 

 curved, and describe about a quarter of a circle. In the upper 

 jaw the concavity of the curve is directed outward ; in the 



1 CLXXX, p. 431. 



Dentition, Phascolomys fuscus, >i nut. size. 



