CARAPACE. 



403 



Trionychoidea and the Athecae in which the skin is soft, by a 

 horny epidermal exoskeleton consisting of horny plates which 

 constitute the so-called " tortoiseshell," and which have an 

 arrangement roughly resembling that of the subjacent bony 

 plates of the shell. 



The carapace (Fig. 224), beneath which the head and tail can 

 often be retracted, owes its origin to the formation of bony plates 

 in the dermis, some of which become attached to the internal 

 skeleton. Allowing for some variation in details, it is constituted 

 as follows : there is a median row of usually eight neural plates, 

 which are fused (Fig. 224) 

 to the spinous processes 

 of eight of the thoracic 

 vertebrae (2nd to 9th) ; 

 in front of the first neural 

 plate andsuturally joined 

 to it, is a large nuchal 

 plate (Fig. 224, Nu), 

 which lies over and is 

 joined by ligament to 

 the neural spine of the 

 last (8th) cervical verte- 

 bra. Behind the eighth 

 neural are the median 

 pygal (Py) plates (one to 

 three in number), of 

 which the first two are 

 united to the eighth cos- 

 tal (08) and the last to 

 the hinder marginal. The ribs of the second to the eighth 

 thoracic vertebrae, distinguished from the first and last ribs by 

 their greater length, are fused with the broad, transversely 

 arranged costal plates of the carapace (C). The costal plates 

 are joined with one anotlier by sutures, and present the special 

 peculiarity of giving off broad transverse plate?, which arch 

 over the muscles of the back and join the neural plates (Fig. 225). 

 The ribs project beyond the costal plates and end, each of them, 

 in one of the marginal plates {M), of which there are usually 

 eleven pairs and a median posterior. The marginals form the 

 boundary of the carapace. The first marginal on each side is 



Fig. 224. — Dorsal view of the carapace of Chelone 

 midixs (from Huxley). Nu nuchal, Py pygal 

 plates ; M marginal plates ; R ribs ; ; 1-8 neural 

 plates ; C1-C8 costal plates. 



