Zoological Society. 133 



manent spurious molar 'which succeeds the corresponding 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 Ma- 

 cropus 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 an- 

 terior ones to the vis a tergo of their successors. 



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 | ; canines g ; praemolares ~ ; molares j^ : = 24. 



The incisors, moreover, are true dentes 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 subtrihedral 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 



