THE TURTLE. 205 



The relation between the plates which make up the 

 shell can be better understood by separating them. If 

 the specimen which was used for dissection, after being 

 freed from most of its soft parts, be dipped into boiling 

 water for a few minutes, the horny epidermal (tortoise- 

 shell) plates may be readily stripped off. If then it be 

 boiled thoroughly, the marginal nuchal and pygal dermal 

 plates may be removed. There will then remain only the 

 endoskeleton, forming still a considerable part of the con- 

 cave surface of the carapace. 



II. Endoskeletal Parts. On the convex surface, as it 

 is now exposed, there will be seen markings correspond- 

 ing to the edges of the epidermal plates which have been 

 stripped off. Observe that these markings do not at all 

 correspond to the outlines of the plates of bone they 

 overlie. These plates of bone are arranged in longitudi- 

 nal series as follows : 



There is a central row of eight plates formed by the 

 lateral expansion of the neural spines of eight of the ver- 

 tebrae, and hence called neural plates. 



There are two lateral rows, each of eight plates, formed 

 by the expansion of eight pairs of ribs, and hence called 

 costal plates. 



III. The Spinal Column. Study now the arrangement 

 of bones, as seen on the concave side of the carapace. 

 The spinal column marks the median line. In it are four 

 distinguishable regions : 



1. A cervical region, extending from the head to the 

 shoulder girdle. The cervical vertebrae bear no ribs, and 

 are not attached directly to the carapace. 



2. A dorsal region, of rib-bearing vertebrae, cob'ssified 

 with the carapace. 



3. A sacral region, of two vertebrae, not attached directly 

 to the carapace, bearing ribs which unite distally to form 

 a facet for articulation with a bone of the pelvic arch. 



