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; {119} in _Myrmecobius_ (alone) is the third toe the longest; and no great difference can be detected between the third and fourth toes in the case of the genera _Phascologale_, _Didelphys_, and some others. Professor Leche compares the predominance of the fourth toe with the hyperphalangeal condition in the fourth toe of the embryo Crocodile, and considers it an archaic feature, not surpassed by the ancient characteristics of the Monotremata. Again it has been pointed out that in _Phascologale_ and _Perameles_, the epistropheus (axis vertebra) has a separate rib as in _Ornithorhynchus_. In the third place, the likeness of the teeth of _Myrmecobius_ to those of _Ornithorhynchus_ is an argument in the same direction, which is furthermore supported by the great age (Mesozoic) of the Metatherian group, if we are right in regarding those extinct creatures as Marsupials.