THE SKELETON IN MAMMALIA. EXOSKELETON. 417 



In the hedgehogs, porcupines and Echidna certain of the 

 hairs are modified and greatly enlarged, forming stiff spines. 

 Similar spines occur in the young of Centetes, and in Acan- 

 thornys among the Muridae. 



Several other forms of epidermal exoskeleton are met with 

 in mammals, including : 



(a) Scales. These overlie the bony scutes of armadillos 

 and occur covering the tail in several groups of mammals, such 

 as beavers and rats. In the Manidae the body is covered by 

 flat scales which overlap. 



(b) The horns of Bovine Ruminants. These, which must 

 on no account be confused with antlers, are hollow cases of 

 hardened epidermis fitting on to bony outgrowths of the frontals. 

 In almost every case they are unbranched structures growing 

 continuously throughout life, and are very rarely shed entire. 

 In the Prongbuck A ntilocapra however they are bifurcated and 

 are periodically shed. Horns are nearly always limited to a 

 single pair, but the four-horned antelope Tetraceros has two 

 pairs, the anterior pair being the smaller. 



(c) The horns of Rhinoceroses. These are conical 

 structures composed of a solid mass of hardened epidermal 

 cells growing from a cluster of long dermal papillae. From 

 each papilla there grows a fibre which resembles a thick hair, 

 and cementing the whole together are cells which grow from 

 the interspaces between the papillae. These fibres differ from 

 true hairs in not being developed in pits in the dermis. 

 Rhinoceros horns may be either one or two in number, and 

 are borne on the fronto-nasal region of the skull. They vary 

 much in length, the longest recorded having the enormous 

 length of fifty-seven inches. 



(d) Nails, hoofs and claws. In almost all mammals 

 except the Cetacea, these are found terminating the digits of 

 both limbs. Nails are more or less flattened structures, 

 claws are pointed and somewhat curved. In most mammals 



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