The _Mammals_ of the Jurassic period are known to us by a number of small forms which occur in the "Stonesfield Slate" (Great Oolite) and in the Purbeck beds (Upper Oolite). The remains of these are almost exclusively separated halves of the lower jaw, and they indicate the existence during the Oolitic period in Europe of a number of small "Pouched animals" (_Marsupials_). In the horizon of the Stonesfield Slate four genera of these little Quadrupeds have been described--viz., _Amphilestes, Amphitherium, Phascolotherium_, and _Stereognathus_. In _Amphitherium_ (fig. 183), the molar teeth are furnished with small pointed eminences or "cusps;" and the animal was doubtless insectivorous. By Professor Owen, the highest living authority on the subject, _Amphitherium_ is believed to be a small Marsupial, most nearly allied to the living Banded Ant-eater (_Myrmecobius_) of Australia (fig. 158). _Amphilestes_ and _Phascolotherium_ (fig. 184) are also believed by the same distinguished anatomist and palæontologist to have been insect-eating Marsupials, and the latter is supposed to find its nearest living ally in the Opossums (_Didelphys_) of America. Lastly, the _Stereognathus_ of the Stonesfield Slate is in a dubious position. It may have been a Marsupial; but, upon the whole, Professor Owen is inclined to believe that it must have been a hoofed and herbivorous Quadruped belonging to the series of the higher Mammals (_Placentalia_). In the Middle Purbeck beds, near to the close of the Oolitic period, we have also evidence of the existence of a number of small Mammals, all of which are probably Marsupials. Fourteen species are known, all of small size, the largest being no bigger than a Polecat or Hedgehog. The genera to which these little quadrupeds have been referred are _Plagiaulax, Spalacotherium, Triconodon_, and _Galestes_. The first of these (fig. 184, 4) is believed by Professor Owen to have been carnivorous in its habits; but other authorities maintain that it was most nearly allied to the living Kangaroo-rats (_Hypsiprymnus_) of Australia, and that it was essentially herbivorous. The remaining three genera appear to have been certainly insectivorous, and find their nearest living representatives in the Australian Phalangers and the American Opossums.