The Upper Trias of Germany has afforded to Professor Pleininger two teeth of a small mammal, to which the name of _Microlestes antiquus_ has been given, under the impression that it was carnivorous, though it now seems more likely that it was a vegetable feeder. In rocks of nearly the same age in America, Emmons found a jaw-bone of another species (_Dromatherium sylvestre_), which has been supposed to be a near ally of the existing _Myrmecobius fasciatus_ of Australia (Figs. 166, 167). In the Stonesfield slate, a member of the English Jurassic, several other species have been found (Fig. 168), and a still larger number in the fresh-water beds of the Upper Purbeck. Marsh has obtained many others from the Jurassic of America. None appear to have yet been found in the Cretaceous, but they reappear in the Eocene Tertiary, and continue to the modern time. Their absence in the Cretaceous is probably a mere accident, and they present an illustration of a very permanent type little changed since its first introduction. Lyell enumerates in all thirty-three species from the Mesozoic, all of them of small size, and all more or less nearly related to existing Australian Marsupials, though differing much among themselves, and including both carnivorous and herbivorous forms (Fig. 169). Marsh has recently suggested a somewhat new interpretation of these interesting mammalian remains.[79] He considers them divisible into two groups, one allied to the modern Insectivora (Moles, Shrews, Hedgehogs, &c.), but of generalized forms. For these he constitutes a new order (_Pantotheria_, Marsh). The other group is less numerous and is Marsupial (_Allotheria_, Marsh). The jaws in Figs. 166 and 168 belong to the former group, that in Fig. 169 to the latter. We should thus have both placental and Marsupial mammals in the Mesozoic. Marsh remarks that the descent of these different types from a common ancestry would require us to trace mammals back into the Palæozoic, that is, on the doctrine of gradual evolution. [Illustration: FIG. 166.—Jaw of _Dromatherium sylvestre_ (Emmons). From the Trias of North Carolina.] [Illustration: FIG. 167.—_Myrmecobius fasciatus._ A modern Australian marsupial, allied to Mesozoic species.] So soon as the palæontologist passes from the Upper Cretaceous to the Eocene, he finds himself in the domain of the placental mammals, which appear in numerous and large species, and this, not merely in one region, but in every part of the world in which these deposits are known to exist. [Illustration: FIG. 168.—Jaw, and enlarged molar of _Phascolotherium Bucklandi_. Stonesfield slate. England.—After Phillips.] [Illustration: FIG. 169.—_Plagiaulax Becklesii._ Jaw, and pre-molar enlarged, showing flat surface, with ridges.—Purbeck.]