THE PLIOCENE PERIOD. 323 



and very generally into their present genera, and were also represented 

 by many genera and numerous species which are now extinct. A list 

 of Pliocene families would be little more than a catalogue of those 

 now living. The evolution of the horse was advanced to the existing 

 genus, Equus. The hornless rhinoceros continued in North America 

 till near the close of the period, and then passed away. The horned 

 branch flourished in the Old World, while the tapir disappeared from 

 Europe. Giraffes and giraffe-like animals (Samotherium , Helladotherium, 

 Sivatherium, Bramatherium, Vishnutherium), some of them of gigantic 

 dimensions, invaded southern Europe and Asia, coming probably from 

 Africa. The three last named have been found in the great Siwalik 

 formation of India. 



The giants of the period were the proboscidians. The Dinothe- 

 rium may be regarded as an aberrant side branch that suffered the 

 usual fate of such branches — early extinction. It was somewhat widely 

 distributed in Europe and has been found in India, but is not known 

 to have reached America. The mastodons seem to have occupied 

 all the continents during the Pliocene, but it is doubtful whether the 

 elephant reached the American continent before the Pleistocene. 

 Some of the early mastodons had tusks in the lower as well as upper 

 jaw (Tetrabelodon), but the most of the Pliocene species had tusks in 

 the upper jaw only, in- the adult state (Dibelodon) . The mastodons 

 were very closely related to the elephants, and are most conveniently dis- 

 tinguished by the teeth, the molars of the former being crowned by 

 conical tubercles, while those of the latter are marked by transverse 

 folds of enamel, separated by cement (Figs. 466 and 467). The ele- 

 phants appear to have flourished abundantly in Europe, and with the 

 associated rhinoceroses and hippopotamuses gave to the European 

 fauna an African aspect. 



The carnivores of both continents flourished and perhaps gained 

 somewhat upon the herbivores; at any rate they put a severe tax on 

 the herbivores, forcing still further adaptations in the line of alert- 

 ness, sagacity, speed, and defense, and gaining similar qualities them- 

 selves. Besides most of the existing genera, the ferocious " saber- 

 toothed tiger " (Machccrodus) and some other extinct forms still existed. 

 The rodents appear to have held about their present place relatively. 



Supreme interest attaches to the development of the primates in 

 this period, but as yet the data are limited and are likely, to remain 



