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).
Fig. 58.—Sagittal section of brain of Rock Wallaby (Petrogale penicillata). ant.com, Anterior commissure; cbl, cerebellum; c.mam, corpus mammillare; c.qu, corpora quadrigemina; crur, crura cerebri; epi, epiphysis, with the posterior commissure immediately behind it; f.mon, position of foramen of Monro; hip.com, hippocampal commissure, consisting here of two layers continuous behind at the spleneium, somewhat divergent in front where the septum lucidum extends between them; hypo, hypophysis; med, medulla oblongata; mid.com, middle commissure; olf, olfactory lobe; opt, optic chiasma; vent. 3, third ventricle. (From Parker and Haswell's Zoology.)
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 dentition of the group is on the whole composed of more numerous individual teeth than in the typical Eutheria; but we have exceptions like the Whales, the Armadillo Priodontes, and the Manatee; or better, because free from the suspicion of secondary multiplication, Otocyon and occasionally (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 importance 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 development 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;
