416 LAND MAMMALS IN THE WESTERN HEMISPHERE 



forms. No record of the presence of the family in the southern 

 continent has been found in beds older than the Pleistocene, 

 but in view of the degree of specialization which they have 

 there undergone, it is probable that the immigration took place 

 in the Pliocene. 



SECTION CAVICORNIA. HOLLOW-HORNED RUMINANTS 



In the animals of this second and far larger section of the 

 Pecora there are bony outgrowths of the skull, from the frontal 

 bones, outgrowths which are permanent and non-deciduous ; 

 these are the horn-cores, which are tapering and unbranched. 

 The horn-core is, in turn, covered with a sheath of horn, like- 

 wise unbranched and permanent, but growing from year to 

 year until the maximum size is attained, a process which is 

 familiarly illustrated in the growth of a calf. Among Recent 

 Cavicornia there is but one exception to the rule that the 

 horny sheath is non-deciduous and unbranched and that one is 

 the Prong Buck {Antilocapra americana). In the Cavicornia it 

 is the very general rule that both sexes are horned, though the 

 females commonly have smaller horns and in several genera 

 of antelopes the does are hornless. There is almost as great 

 variety in the shape and size of the horn as of the antler ; we 

 find small, medium-sized and enormously large horns, which 

 may be straight, simply curved, complexly curved, spiral, 

 lyrate or twisted. The antelopes have many types of horns, 

 as have the sheep and goats, the oxen, buffaloes and bisons ; 

 but only a few of them are exemplified in the western hemi- 

 sphere, which now, as in the preceding geological periods, is 

 singularly poor in representatives of the Pecora. 



9, 10. Antilopidce and Antilocapridw. Antelopes 



Two very different kinds of antelopes are found in North 

 America at the present time ; one of them, the erroneously 

 named Rocky Mountain Goat {Oreamnos montanus), is evi- 

 dently a late immigrant from the Old World, and fossil remains 



