THE ALLIES OF THE SQUIRREL : MAMMALIA 415 



and in oxen, sheep, and goats, are of similar structure. They 

 are usually found in both sexes and are never shed. In the 

 deer family (Fig. 221) the horns, called antlers, consist of 

 outgrowths of bone covered each year during the period of 

 growth with a sensitive skin called " the velvet." When 

 the annual growth is completed the supply of blood to the 

 antlers ceases and the velvet peels off, leaving the bone bare. 

 After a time the antlers separate from the skull and are shed. 

 In most deer this takes place annually. In some members of 

 the family the antlers remain simple throughout life ; in 



Fi<;. 220. Head of Rhinoceros 

 (American Museum of Natural History) 



others they become much branched in successive annual 

 growths (Fig. 221). It is interesting to note that, in a broad 

 way, this was the order of development of antlers in geo- 

 logical time, the earliest deer of which we have any knowl- 

 edge being without them. Horns and antlers are used in the 

 battles of the males for the possession of the females, as well 

 as for defense of themselves and their band. The presence 



