18 



Tribe V. RHIZOPHAGA. 



The characters of this tribe are taken from the stomach, which is 

 simple in outward form, but complicated within by a large cardiac 

 gland ; and from the caecum, which is short and wide, with a vermi- 

 form appendage. 



Genus Phascolomys. 



In its heavy shapeless proportions, large trunk, and short equably 

 developed legs, the Wombat offers as great a contrast to the Kan- 

 garoos as does the Koala, which it most nearly resembles in its ge- 

 neral outward form and want of tail. But in the more important 

 characters afforded by the teeth and intestinal canal the Wombat 

 differs more from the Koala than this does from either the Phalan- 

 gers or Kangaroos. The dental system presents the extreme de- 

 gree of that degradation of the teeth intermediate between the 

 front incisors and true molares which we have been tracing from 

 the Opossum to the Kangaroos : not only have the functionless 

 spurious molares and canines now totally disappeared, but also the 

 posterior incisors of the upper jaw, which we have seen in the Po- 

 toroos to exhibit a feeble degree of development as compared with 

 the anterior pair; these in fact are alone retained in the denti- 

 tion of the present group, which possesses the fewest teeth of any 

 Marsupial animal. The dental formula of the Wombat, is thus re- 

 duced both in number and kind to that of the true Rodentia : 



Incisors I ; canines ^ ; prsemolares J^ ; molares |^: = 24. 



The incisors, moreover, are true denies scalprarii, with persistent 



pulps, but are inferior, especially in the lower jaw, in their relative 



length, and curvature, to those of the placental Glires : they present 



• a subtriliedral figure, and are traversed by a shallow groove on their 



inner surfaces. 



The spurious molares present no trace of that compressed struc- 

 ture which characterizes them in the Koala and Kangaroos : but have 

 a wide, oval, transverse section : those of the upper jaw being tra- 

 versed on the inner side with a slight longitudinal groove. The 

 true molares have double the size of the spurious ones : 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 inwards. The 

 inferior molares are in like manner divided into two trihedral portions, 

 but the intervening groove is here external, and one of the faces of 

 each prism is turned inwards. All the grinders are curved, and de- 

 scribe about a quarter of a circle ; in the upper jaw the concavity 

 of the curve is directed outwards, in the lower jaw inwards. The 

 false and true molares like the incisors have persistent pulps, and are 

 consequently devoid of true fangs : in which respect the Wombat 

 differs from all other Marsupials, and resembles the extinct Toxodon, 

 the dentigerous Brutu, and herbivorous Rodentia. 



Although none of the Marsupialia possess teeth composed of an 

 intermixture of layers of ivory, cement and enamel through the body 



