174 THE ANATOMY OF YERTEBKATED ANIMALS. 



forms the anterior median boundary of tlie carapace. This 

 nuchal plate sends down from its under-surface a median pro- 

 cess, which is joined by ligament with the expanded neural 

 spine of the eighth cervical vertebra. Behind the eighth neu- 

 ral plate, three other median pygal plates (Fig. 63, Py) suc- 

 ceed one anotlier. The anterior tw^o of these are united by 

 sutures with the eighth neural and costals, and with one an- 

 other ; but the third is connected externally only with the 

 marginal plates. All three are perfectly distinct from the sub- 

 jacent vertebra3. 



The sides of the carapace are completed, between the 

 nuchal and pygal plates, by eleven marginal plates (Fig. G3, 

 31) on each side. Eight of these receive the ends of the ribs 

 of the second to the ninth dorsal vertebra?, in the way already 

 described. 



There is no doubt that the nuchal, the pygal, and the mar- 

 ginal plates of the carapace are membrane-bones, developed 

 in the integument, quite independently of either the vertebr86 

 or the ribs. But it appears that the neural plates and the 

 costal plates exist, as expansions of the cartilages of the neu- 

 ral spines and ribs of the primitive vertebra?, before ossifica- 

 tion takes place. This being the case, the neural and costal 

 plates are vertebral and not dermal elements, however similar 

 they may seem to be to the nuchal, pygal, and marginal plates. 

 But this ultimate similarity of bones of totally distinct origin 

 is not more remarkable here than in the case of the skull, 

 where the parietal and frontal bones stand in the same rela- 

 tion to the supra-occipital bone as the nuchal and pygal plates 

 do to the neural plates of the carapace. 



There are no sternal ribs, and no trace of a true sternum 

 has yet been discovered in the Clielonia. The plastron is 

 wholly composed of membrane-bones, w'hich are developed in 

 the integument, and lie, in part, in front of, and, in part, be- 

 hind, the umbilicus of the foetus. The latter, at least, there- 

 fore belong to the abdomen, and the plastron is a thoracico- 

 abdominal structure. 



In the turtle the plastron consists of nine pieces — one me- 

 dian and anterior, four lateral and paired (Fig. 64). These 

 f)ieces may be named — the median, entoplastron ; the first 

 ateral, exAplastron ^ the second, hyoxtloatron ^ the third, hypo-- 

 plastron y and the iouvih^ xip>hiplastr on. ^ The e92toplastro9i 



* Believing tlie plastron to answer to tlie sternum of other Verfciraia, 

 anatomists have termed these elements of the plastron entosternnm^ epister- 

 num^ hyostemum^ Jiyposternum^ and xipMsternum. 



