326 HISTORICAL PALEONTOLOGY. 



and Azof Seas, are Pliocene deposits known as the " Aralo- 

 Caspian " beds. The fossils in these beds are partly fresh- 

 water, partly marine, and partly intermediate in character, and 

 they are in great part identical with species now inhabiting 

 the Caspian. The entire formation appears to indicate the 

 former existence of a great sheet of brackish water, forming an 

 inland sea, like the Caspian, but as large as, or larger than, the 

 Mediterranean. 



6. In the United States, strata of Pliocene age are found in 

 North and South Carolina. They consist of sands and clays, 

 with numerous fossils, chiefly Molluscs and Echinoderms. 

 From 40 to 60 per cent of the fossils belong to existing 

 species. On the Loup Fork of the river Platte, in the Upper 

 Missouri region, are strata which are also believed to be refer- 

 able to the Pliocene period, and probably to its upper division. 

 They are from 300 to 400 feet thick, and contain land-shells, 

 with the bones of numerous Mammals, such as Camels, Rhino- 

 ceroses, Mastodons, Elephants, the Horse, Stag, &c. 



As regards the life of the Pliocene period, there are only 

 two classes of organisms to which our attention need be 

 directed namely, the Shell-fish and the Mammals. So far as 

 the former are concerned, we have to note in the first place 

 that the introduction of new species of animals upon the globe 

 went on rapidly during this period. In the Older Pliocene 

 deposits, the number of shells of existing species is only from 

 40 to 60 per cent ; but in the Newer Pliocene the pro- 

 portion of living forms rises to as much as from 80 to 

 95 per cent. Whilst the Molluscs thus become rapidly mo- 

 dernised, the Mammals still all belong to extinct species, 

 though modern generic types gradually supersede the more 

 antiquated forms of the Miocene. In the second place, there 

 is good evidence to show that the Pliocene period was one in 

 which the climate of the northern hemisphere underwent a 

 gradual refrigeration. In the Miocene period, there is evi- 

 dence to show that Europe possessed a climate very similar 

 to that now enjoyed by the Southern United States, and cer- 

 tainly very much warmer than it is at present. The presence 

 of Palm-trees upon the land, and of numerous large Cowries, 

 Cones, and other shells of warm regions in the sea, sufficiently 

 proves this. In the Older Pliocene deposits, on the other 

 hand, northern forms predominate amongst the Shells, though 

 some of the types of hotter regions still survive. In the Newer 

 Pliocene, again, the Molluscs are such as almost exclusively 

 inhabit the seas of temperate or even cold regions ; whilst if 

 we regard deposits like the " Bridlington Crag" and "Chilles- 



