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.

Finally, it is interesting to notice in how many respects the Jurassic fauna of Western Europe approached to that now inhabiting Australia.  At the present day, Australia is almost wholly tenanted by Marsupials; upon its land-surface flourish Araucarioe and Cycadaceous plants, and in its seas swims the Port-Jackson Shark (Cestracion Philippi); whilst the Molluscan genus Trigonia is nowadays exclusively confined to the Australian coasts.  In England, at the time of the deposition of the Jurassic rocks, we must have had a fauna and flora very closely resembling what we now see in Australia.  The small Marsupials, Amphitherium, Phascolotherium, and others, prove that the Mammals were the same in order; cones of Araucarian pines, with tree-ferns and fronds of Cycads, occur throughout the Oolitic series; spine-bearing fishes, like the Port-Jackson Shark, are abundantly represented by genera such as Acrodus and Strophodus; and lastly, the genus Trigonia, now exclusively Australian, is represented in the Oolites by species which differ little from those now existing.  Moreover, the discovery during recent years of the singular Mud-fish, the Ceratodus Fosteri in the rivers of Queensland, has added another and a very striking point of resemblance to those already mentioned; since this genus of Fishes, though preeminently Triassic, nevertheless extended its range into the Jurassic.  Upon the whole, therefore, there is reason to conclude that Australia has undergone since the close of the Jurassic period fewer changes and vicissitudes than any other known region of the globe; and that this wonderful continent has therefore succeeded in preserving a greater number of the characteristic life-features of the Oolites than any other country with which we are acquainted.

LITERATURE.

The following list comprises some of the more important sources of information as to the rocks and fossils of the Jurassic series:—­