118 MOLARS OF EUTHERIA : CHAP. 
of the Marsupials as “singularly specialised characters,” im no 
way intermediate in character. This view apples 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 
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 
cmam vents Eutheria; but we have 
Fic. 58.—Sagittal section of brain of Rock Wallaby exceptions like the 
(Petrogale penicillata). ant.com, Anterior commis-  yy7},.7,. wees 
sure ; cb/, cerebellum ; ¢.mam, corpus mammillare ; Whales, the Arma- 
e.qv, corpora quadrigemina ; erwr, crura cerebri dillo Priodontes, and 
epi, epiphysis, with the posterior commissure im- fei Maine ; ; 
mediately behind it ; fmon, position of foramen of 1€ Manatee, ol 
Monro ; hip.com, hippocampal commissure, consist- better, because free 
ing here of two layers continuous behind at the oe ip 
spleneium, somewhat divergent in front where the from the suspicion of 
Segums lucidum extends between them ; hypo, hypo- secondary multiplica- 
physis ; med, medulla oblongata ; mid.com, middle — , 
commissure ; olf, olfactory lobe ; opt, optic chiasma ; tion, Otocyon and ocea- 
Cae ventricle. (From Parker and Haswell’s sionally (according to 
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 ; 
opt hip.com 
