io86 



CLASS REPTILIA. 



femoral shields dividing the normal pairs of plastral shields ; and it 

 is probable that this may be regarded as the archaic type. Similarly 

 in a Chelonian, mentioned below under the name of Tropidemys, 

 there is a median series of intervertebral shields dividing the normal 

 vertebrals into two lateral rows. In some cases inframarginal 

 shields separate the marginal from the plastral shields. 



The bones of the carapace, although following the same general 

 arrangement, do not, as will be seen from the figures, by any means 



Fig. 1009 bis. — Upper surface of the carapace of Thalassochelys caretta, with the epidermal 

 shields removed. The dark lines in the middle region indicate the boundaries of the shields, and 

 the sinuous ones the sutures between the bones, nu, Nuchal bone ; py, Pygal do. ; the bone 

 above this marked n and the next are the suprapygals ; above these, the eight neurals, of which 

 the first is marked n ; c, c, Costal bones ; m, m, Marginal do. ; v, v, Vertebral shields. Much 

 reduced. (After Owen.) 



correspond with the overlying shields. In those forms with a well- 

 developed carapace there are typically (fig. 1017) eight median 

 neural bones formed by the expansion of the spines of the dorsal 

 vertebrae (fig. 10 10) ; these being preceded by a nuchal, and fol- 

 lowed by two or more suprapygals and a pygal, all of which have 

 no connection with the vertebrae. While, however, the nuchal is a 

 cartilage bone, the pygal and suprapygals are of purely dermal origin. 

 On either side of this median row is a series of from seven to nine 



