THE CARAPACE OF THE CHELONIA. 173 



rertebra is shorter, from before backward, than its centrum j 

 and the neural arch of the second dorsal vertebra extends for- 

 ward and overlaps the centrum of the first, for the space thus 

 left unoccupied. The rib of the second vertebra is also carried 

 forward, and articulates not only with its own centrum and 

 neural arch, but with the posterior edge of the centrum of the 

 first vertebra. 



These arrangements are repeated by the other dorsal ver- 

 tebras and ribs, up to the ninth inclusive ; but, in the tenth, 

 the neural arch occupies only the anterior half of the centrum 

 of its own vertebra, and the rib is very small, and has no cos- 

 tal plate. 



The union of the neural and costal plates of the eight dor- 



riG. 68. — Dorsal view of the carapace of C?iel<mf> midas : Ku^ nuchal plate ; J/", marginal 

 plates; i2, ribs; 1-8, neural plates ; (7, 1-C.8, costal plates; Py, pygal plates. 



sal vertebrae, from the second to the ninth inclusively, gives ' 

 rise to the principal part of the carapace, or dorsal moiety of 

 the bony shell of the Chelonian. The first and the tenth dor- 

 sal vertebree contribute nothing to the carapace, their small 

 ribs merely becoming attached to the costal plates behind and 

 before them. 



In front of the first neural plate, and joined with it by a 

 serrated suture, lies a large nuchal plate (Fig. 63, Nu), which 



