That there was a continuity of dry land between Europe and North America during the Miocene epoch, appears to me to be a necessary consequence of the fact that many genera of terrestrial mammals, such as _Castor, Hystrix, Elephas, Mastodon, Equus, Hipparion, Anchitherium, Rhinoceros, Cervus, Amphicyon, Hyoenarctos_, and _Machairodus_, are common to the Miocene formations of the two areas, and have as yet been found (except perhaps _Anchitherium_) in no deposit of earlier age. Whether this connection took place by the east, or by the west, or by both sides of the Old World, there is at present no certain evidence, and the question is immaterial to the present argument; but, as there are good grounds for the belief that the Australian province and the Indian and South-African sub-provinces were separated by sea from the rest of Arctogaea before the Miocene epoch, so it has been rendered no less probable, by the investigations of Mr. Carrick Moore and Professor Duncan, that Austro- Columbia was separated by sea from North America during a large part of the Miocene epoch. It is unfortunate that we have no knowledge of the Miocene mammalian fauna of the Australian and Austro-Columbian provinces; but, seeing that not a trace of a Platyrrhine Ape, of a Procyonine Carnivore, of a characteristically South-American Rodent, of a Sloth, an Armadillo, or an Ant-eater has yet been found in Miocene deposits of Arctogaea, I cannot doubt that they already existed in the Miocene Austro-Columbian province. Nor is it less probable that the characteristic types of Australian Mammalia were already developed in that region in Miocene times. But Austro-Columbia presents difficulties from which Australia is free; _Cantelidoe_ and _Tapirdoe_ are now indigenous in South America as they are in Arctogaea; and, among the Pliocene Austro-Columbian mammals, the Arctogaeal genera _Equus, Mastodon,_ and _Machairodus_ are numbered. Are these Postmiocene immigrants, or Praemiocene natives? Still more perplexing are the strange and interesting forms _Toxodon, Macrauchenia, Typotherium_, and a new Anoplotherioid mammal (_Homalodotherhon_) which Dr. Cunningham sent over to me some time ago from Patagonia. I confess I am strongly inclined to surmise that these last, at any rate, are remnants of the population of Austro-Columbia before the Miocene epoch, and were not derived from Arctogaea by way of the north and east.