CENOZOIC ERA: AGE OF MAMMALS 607 



mammals of the time, it would eventually have dropped its side 

 toes and have walked on one toe like the modern horse (p. 608). 



(3) The true rhinoceroses constitute the third group, but with 

 two exceptions (Diceratherium and Teleoceros), none of the North 

 American forms had horns. None of this family are known to have 

 lived in North America after the Pliocene, but members of the group 

 were able to adapt themselves to the vicissitudes of the closing days 

 of the Tertiary and roamed over Europe and Asia, some (woolly 

 rhinoceros) being adapted even to the cold climate of northern Asia, 

 as a carcass found frozen in the ice of northern Siberia shows. 



The principal changes which the true rhinoceros group underwent 

 in its history are (1) an increase in bulk, (2) a reduction in the 

 toes from four on the fore foot to three on all the feet, (3) the develop- 

 ment of horns, and (4) the development of somewhat better teeth. 



REFERENCES FOR RHINOCEROSES 



International Encyclopedia, — Rhinoceros. 



Osborn, H. F., — Age of Mammals, p. 272. 



Osborn, H. F., — Phylogeny of the Rhinoceroses of Europe: Bull. Am. Mus. Nat. 

 Hist., Vol. 13, No. 5, 1900, pp. 229-267. 



Scott, W. B., — A History of Land Mammals in the Western Hemisphere, pp. 326-353. 



(A description of Arsinoitherium, an interesting Eocene animal with somewhat the 

 appearance of a rhinoceros but unrelated, is to be found in Osborn, Age of Mam- 

 mals, p. 202; and in Lankester, E. R., Extinct Animals, pp. 152-154.) 



Tapirs. — It is interesting to find an animal living in the present 

 which still retains the characters of animals that are more typical of 

 Eocene and Miocene times before differentiation became marked. 



The teeth of tapirs are short-crowned and differ but slightly from 

 those of their Miocene ancestors of Nebraska and South Dakota. 

 They are odd-toed ungulates (perissidactyls), with four toes (Fig. 

 541 A) on the fore foot (the weight being on the third toe), and three 

 on the hind foot, the fourth toe of the fore foot being small. The 

 Eocene ancestors of the tapirs graded almost insensibly into those 

 of the horse and rhinoceros. 



Tapirs live in marshes or dense forests in proximity to water, 

 occupying a place in nature in which there is little mammalian 

 competition. They had a wide distribution in the past and are an 

 illustration of a once abundant race nearly exterminated but still 

 struggling for existence where competition happens to be least severe 

 in their particular case. Their present occurrence only in South 



CLELAND GEOL. — 39 



