ii8 



MOLARS OF EUTHERIA 



of the Marsupials as " singularly specialised characters," in uo 

 way intermediate in character. This view applies also to the 

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

 group. But the inipossiljility of using this last character as 

 one of any importance has been shown liy the discovery of 

 rudiments of it in emhryos of undoubtedly Eutherian mammals 

 (see p. 18). 



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



the Marsupials as 

 dividing them from 

 the higher mammals 

 than was formerly the 

 case. The total denti- 

 tion of the group is 

 on the whole com- 

 posed of more numer- 

 ous individual teeth 

 than in the typical 

 Eutheria. ; 1 )ut we have 



cbl 



ant.coTTi 



Tned 



CmCLTTl 



vent.s 



Fig. 58. — Sagittal section 

 ( Petrogale penicUlata). 



sure ; cbl, cerebellum ; c.mam, corpus maniinillare : 

 '■.(/'', corpora quadrigemina ; crvi\ crura cereljri ; 

 I'lii, epiphysis, with the posterior coniniissure ini- 

 ineiliately behind it ; f.iiion, ]insition of foramen of 

 Monro ; liip.cnm, hippocampal commissure, consist- 

 ing liere of two layers continuous behind at the 

 spleneium, somewhat divergent in front whei-e the 

 septnni lucidum extends between them ; hy2m, hypo- 

 l)hysis ; med, meilulla oblon.L,'ata ; mid.com, middle 

 commissure ; olf, olfactory lobe ; opt, optic chiasma ; 

 vent. 3, thinl ventricle. (From Parker and Haswell's 

 Zoology.) 



of lirain of Rock Wallaby exceptions like the 

 nat.com. Anterior commis- -ttt-, i , , . 



Whales, the Arma- 



dillo Friodontes, and 

 the Manatee ; or 

 better, because free 

 from the suspicion of 

 secondary multiplica - 

 tion, Otocyon and occa- 

 sionally (according to 

 Mr. Thomas) Centctes. 

 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 ; 



