In the fossils above enumerated, several species are larger than the largest living ones of the same genera now known in Australia. Fig. 91 of the right side of a lower jaw of a kangaroo (_Macropus atlas_, Owen) will at once be seen to exceed in magnitude the corresponding part of the largest living kangaroo, which is represented in Fig. 92. In both these specimens part of the substance of the jaw has been broken open, so as to show the permanent false molar (_a_, Fig. 91), concealed in the socket. From the fact of this molar not having been cut, we learn that the individual was young, and had not shed its first teeth. The reader will observe that all these extinct quadrupeds of Australia belong to the marsupial family, or, in other words, that they are referable to the same peculiar type of organisation which now distinguishes the Australian mammalia from those of other parts of the globe. This fact is one of many pointing to a general law deducible from the fossil vertebrate and invertebrate animals of times immediately antecedent to our own, namely, that the present geographical distribution of organic _forms_ dates back to a period anterior to the origin of existing _species_; in other words, the limitation of particular genera or families of quadrupeds, mollusca, etc., to certain existing provinces of land and sea, began before the larger part of the species now contemporary with man had been introduced into the earth. Professor Owen, in his excellent “History of British Fossil Mammals,” has called attention to this law, remarking that the fossil quadrupeds of Europe and Asia differ from those of Australia or South America. We do not find, for example, in the Europæo-Asiatic province fossil kangaroos, or armadillos, but the elephant, rhinoceros, horse, bear, hyæna, beaver, hare, mole, and others, which still characterise the same continent. In like manner, in the Pampas of South America the skeletons of Megatherium, Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and other extinct forms, are analogous to the living sloth, armadillo, cavy, capybara, and llama. The fossil quadrumana, also associated with some of these forms in the Brazilian caves, belong to the Platyrrhine family of monkeys, now peculiar to South America. That the extinct fauna of Buenos Ayres and Brazil was very modern has been shown by its relation to deposits of marine shells, agreeing with those now inhabiting the Atlantic.