Burmeister, Moreno, and Ameghino. A few words about this branch of the subject may be added. The oldest formation containing well-preserved remains of mammals yet investigated is in the neighbourhood of Santa Cruz, in Southern Patagonia, where the deposits are about 200 feet in thickness. The exact age of the Santa Cruz beds it is very difficult to determine, but the best authorities consider that they cannot be of earlier date than the Upper Eocene or Oligocene of Europe. Following these in point of time, are the so-called "Patagonian" beds of Patagonia and Uruguay, which are probably of Miocene age. The "Araucanian" formation of Ameghino, which is well developed at Monte Hermoso, near Bahia Blanca, in Southern Argentina, seems to correspond approximately with the older European Pliocene. Finally, the later Pliocene is apparently represented by the Pampas" formation of Argentina and Uruguay. Our knowledge of the extinct mammal-faunas of these beds is mainly due to the efforts of the Argentine palæontologists just mentioned, but a clear résumé of the work done will be found in a recent number of the Geological Magazine (12). In the "Santa Cruz" beds have been found remains of about 120 genera of mammals referable to the following groups: Marsupials. Perissodactyles. Rodents. Among the Marsupials the most prominent forms in this formation are the Opossums (Didelphyida), which are still found all over America, and in Tertiary times appear to have been distributed nearly all over the northern hemisphere. But accompanying these are other forms of the same Order, which seem to be more nearly related to the Australian marsupials of other families; in fact, Ameghino has placed some of them in the (hitherto supposed to be) exclusively Australian family Dasyuridæ. The fossil Edentates of this formation are all of the American section of the group. The Perissodactyle Ungulates are represented by two families (Proterotheriida and Macraucheniida) not very definitely connected with other Perissodactyles, and unknown elsewhere; and the Toxodonts (another group forming a distinct sub-order of the Ungulata) are also quite unrecognised outside South America, where, indeed, they appear to have existed for a short period only. The Rodents of this formation all belong to the Hystricomorphine section of the order, and the Monkeys are all Platyrrhine, so that, except in the case of the Marsupials where the results are to a certain extent doubtful, no clue to the origin of the Neotropical fauna is shown by this extinct fauna. When these results are compared with the Eocene Mammal-fauna of the northern hemisphere, the absence of Artiodactyles, Insectivores, Bats, Carnivores, and Lemurs, is very striking. The "Patagonian" beds contain a Mammal-fauna only to be distinguished from the previous "Santa Cruz" series. by a further differentiation of genera belonging to the same orders without any traces of foreign admixture. When, however, we reach the more recent Araucanian formation, we find, in addition to the Edentates, Toxodonts, and other typical South American forms, a number of foreign intruders, such as Tapirs, Lamas, Elephants (Mastodon), and Wild Dogs (Canis), of an entirely different aspect. There can be little doubt that these animals had migrated F here southwards from North America, where their remains (or those of closely allied species) have been likewise found in the nearly contemporaneous formations of the United States. Moreover, it would seem that in these bygone days, not only did the northern forms move southwards, but that also some of the southern forms emigrated northwards. This is evidenced by the fauna of the so-called "Equus-beds" and "Megalonyx-beds" of a slightly later date in the United States, which contain a composite Mammal - fauna of northern forms mixed with forms usually considered to be exclusively South American— such as the gigantic armadillo-like Glyptodon, the Capybara (Hydrochorus), Toxodon, and others. Finally, in the age of the "Pampas" beds, the peculiar South American Mammal-fauna seems to have reached its culminating-point, and to have far exceeded that of the present day both in number of species and in the size of the individuals. This great increase in size, which is, as a rule, accompanied by an extreme specialisation of individual organs, seems to have had a fatal effect on its possessors, as none of the larger Edentates or Toxodonts appear to have outlived the end of the pampas formation. Along with most of the larger arrivals from the north, such as Mastodon and Equus, they became extinct. All the conclusions to be derived from this much-abbreviated account of the extinct Mammals of South America, confirm in a remarkable way the evidence of the present fauna as to this history of the Neotropical Region. Up to the last period of the Tertiary epoch, South America was certainly isolated from the rest of the world, and the connections with Australia and with Africa, if they ever did exist, must have been previous to this |