THE MIOCENE PERIOD. 289 



for the great wear caused by the dry silicious grasses. 1 It is probably 

 as safe to infer a development of dry grassy plains from this evolu- 

 tion of the horse, as to infer climatic and topographic conditions from 

 plants and other organic adaptations, and hence it is probably safe 

 to interpret the western " basins " as lodgment plains of the subaerial 

 rather than of the strictly lacustrine type, so far as the nature of the 

 deposits leaves the question open. 



The tapirs and rhinoceroses. — The tapirs were but slightly repre- 

 sented, but the rhinoceroses, though the running and swimming branches 

 had dropped away, were a prominent feature in the fauna, The Ameri- 

 can species were still mainly hornless (Aceraiherium) , slight indica- 

 tions of horns appearing in a single genus (Diceratherium) . Two- 

 horned species, however, appeared during the period in Europe. 



The carnivores. — The carnivores were abundant, and had assumed 

 forms referred with some doubt to the living genera Canis, Felis, Mastela, 

 and Putorius. The Canidcc embraced numerous wolves and foxes, 

 the Felidcc, panther-like animals and saber-toothed cats, the Mustelidcr, 

 weasel-like and otter-like forms, and an ancestral coon is recorded. 

 The genera of the Loup Fork horizon were nearly all different from 

 those of the John Day horizon, which indicates rapid evolution. In 

 Europe, in addition to these four families, the bear, civet, and hyena 

 families were represented, thus including the seven existing families 

 of carnivores. 



The rodents were represented much as in the earlier epoch. 

 Neither the insectivores nor the primates appear in the North American 

 record. The development of the plains which favored the horses, 

 deer, and cattle, was obviously unfavorable to the lemuroids. 



The primates in the Old World. — In the Old World, the true apes, 

 Oreopithecus and Dryopithecus, appeared. The former was a rather 

 large annectant form uniting some of the characters of the apes and 

 the monkeys; the latter was a generalized type related to the chim- 

 panzee and gorilla, and about as large as the former. It is the view of 

 some paleontologists that the ancestral branch of the Hominidce must 

 have diverged from its relatives at least as early as this, since, for ana- 

 tomical reasons, it could not well have been derived from the Simiidce, 



1 An excellent recent statement of the evolution of the horse, admirably illustrated, 

 is given by Matthew. Sup. to Am. Mus. Jour., Vol. Ill, No. I, Jan., 1903, Guide 

 Leaflet No. 9. 



