500 ANATOMY OF VERTEBRATES. 



after the horny shields themselves have perished. The cpiderm 

 is disposed upon the head in various forms : in most freshwater 

 Chelonia it is a continuous thin hard layer on the top and sides of 

 the head : in marine and terrestrial Chelonia it is usually in the 

 form of large plates, leaving marginal impressions on the skull, 

 and distinguished by special names : they are more symmetrical in 

 Chelone than in Testudo. The skin of the neck and of the limbs 

 is covered by small contiguous scales : some of these are of 

 larger size on parts of the feet. The epiderm is thickened and 

 condensed into a beak in all Chelonia,, and into claws in most. 

 In Sphargis, however, the claws are replaced by small coriaceous 

 scales : in Chelone only one digit on each foot supports a claw : 

 in Caretta two digits, and in Trionycidce three digits, support 

 claws : in some Tortoises ( Testudo) there are four claws 011 

 each foot : in most Tortoises and Terrapins (Emys) there is an 

 additional claw on the fore-foot: in the heavy Land Tortoises 

 the claws assume the form of hoofs. The epiderm of the tail is 

 usually wrinkled, and covered only by small scales : in the 

 Snapper (Chelydra) it supports a row of hard compressed 

 tubercles ; in some Tortoises the end of the tail has a thick 

 epidermal sheath, which, in the male Cinosternon, is armed with a 

 pointed tip. 



The large imbricate plates of the carapace of Chelone have their 

 fore border imbedded in a matrix of the derm, and here receive 

 their chief increase, the older parts moving backward, and being 

 worn off. The plates with contiguous borders receive increase 

 at their under surface and entire circumference ; the concentric 

 lines of growth may be marks of annual increment : * but these 

 usually show a greater ratio of growth at the front and sides than 

 at the back The old and dull superficial layer is worn away, or 

 thrown off from time to time, leaving the rest of the epidermal 

 shield of a bright colour : the smooth or scaly epiderm of the 

 limbs and neck is usually shed entire. 



The skin of the neck developes fimbriate processes and 

 caruncles in Chelys fimbriata : that of the nose is produced into a 

 short snout in most TrionycidcB. 



The deeper stratum of soft, usually imbricate, epithelial cells 

 of the epiderm are intermixed with pigmental cells, mostly dark 

 brown or black ; abundant pigment-particles are also suspended 

 in an oily fluid, occupying areolar spaces of the deep epiderm, 

 and usually of the brighter yellow, red, or green colours. Such 



1 As conjectured by Agussiz, ccc. vol. i. pt. ii. p. 259. 



