398 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Hemisphere. The deer family has been present in the Northern 

 Hemisphere since the OHgocene (Fig. 164, 3). In the hollow- 

 horned ruminants (antelopes, sheep, oxen, etc.) the unbranched 

 horn covers a solid bony outgrowth of the frontal bone ; it is 

 usually present in both sexes and is never shed, except in the 

 pronghorn antelope of western North America, which species 

 is similarly exceptional in having branched horns. The sheep 

 and goat sub-family dates from the Upper Miocene of Europe ; 

 they had reached Asia and Africa by the Pliocene, but North 

 America only in recent times. Domestic cattle (Bos) have been 

 found in Asia from the Pliocene, in Europe since the Pleistocene, 

 brought in by man, and in America since recent times only. 

 The bison is known from the Pleistocene to the present in North 

 America and Europe. The solid-horned and the hollow-horned 

 ruminants diverged from a common stock in the Oligocene. 



Order p, Sirenia {Sea Cows) 



Aquatic mammals, with a moderate-sized head, a fish-like 

 body, front limbs paddle-like, hind ones absent and a horizon- 

 tally expanded tail fin. Pelvic bones vestigial. 



This order of sea cows, a highly modified offshoot of the ungu- 

 lates, is probably of African origin. It is first recognized from 

 the Eocene of Egypt and the West Indies ; later it occurs also 

 in Europe and America. During the Pliocene and Pleistocene 

 it was abundant upon both the Atlantic and Pacific coasts of 

 North America. At present it is represented by the dugong of 

 the Indian Ocean and the manatee of the rivers of northeastern 

 South America and western Africa. The sea cows and the ele- 

 phants may possibly have a similar origin, both being derived 

 from a form closely allied to the mid-Eocene Moeritherium 

 (Fig. 169). 



Order 10, Cetacea {Whales) 



Aquatic mammals, with a large head, fish-like body, front 

 limbs paddle-like, hind ones absent and a horizontally expanded 

 tail fin. Vestiges of pelvic bones present. 



