I I 8 MOLARS OF EUTHERIA CHAP. 



of the Marsupials as " singularly specialised characters," in no 

 way intermediate in character. This view applies also to the 

 pouch, which, as already stated, distinguishes the adults of that 

 group. But the impossibility of using this last character as 

 one of any importance has been shown by the discovery of 

 rudiments of it in embryos of undoubtedly Eutherian mammals 

 (see p. 18). 



Less stress is laid now upon the existence of four molars in 



the Marsupials as 



hi Pf m dividing them from 



. , 

 _ the higher mammals 



cbl stsfSS^X than was formerly the 



case. The total denti- 



.aril.com . <? . 



tion ol the group is 



on the whole com- 

 posed of more numer- 

 ous individual teeth 



mecl hyao C7 /* than in the typical 



c.mam venl.9 Eutheria; but we have 



FIQ. 58. Sagittal section of brain of Rock Wallaby exceptions like the 



(Petrogale penwittata). ant.com, Anterior commis- -7-u i pa <-!, A rTri , 

 sure ; cbl, cerebellum ; c.mam, corpus manimillare ; 



c.qu, corpora quadrigemina ; cno; crura cerebri ; dillo PriodoiltcS, and 



epi, epiphysis, with the posterior commissiire. ini- , , -..- , 



mediately behind it ; /.mow, position of foramen of til 6 Manatee ; Or 



Monro ; hig.com, hippocampal commissure, consist- better because free 



ing here of two layers continuous behind at the , 



spleueium, somewhat divergent in front where the irom tne Suspicion OI 



septum lucidum extends between them ; ffypo, hypo- secondary multiplica- 



physis ; med, medulla oblongata ; mid.com, middle . 



commissure ; olf, olfactory lobe ; opt, optic chiasma ; tion, Otocyon and OCCa- 



vent. 3, third ventricle. (From Parker and Haswell's 8 i ona U y (according to 



Zoology.) J ^ 



Mr. Thomas) Centetes. 

 In the last two there are at least sometimes four molars. 



On the other hand, a few archaic characters of some import- 

 ance crop up here and there among the Marsupials, which are 

 sometimes held to point to a primitive ancestry. It has been 

 remarked that in Marsupials it is the fourth toe which is dominant 

 in size, whereas in Ungulates it is the third. An attempt has 

 been made to explain this on the view (reasonable enough in 

 itself) of a tree-living ancestry for the group. A greater develop- 

 ment of the fourth toe is, however, by no means a necessary 

 character of arboreal creatures ; the Primates themselves are an 

 exception. Nor is this prevalence universal among the Marsupials ; 



