THE PLASTRON OF THE CHELONIA. 201 



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

 which forms the anterior median boundary of the carapace. 

 This nuchal plate sends down from its under-surface a 

 median process, which is joined by ligament with the ex- 

 panded neural spine of the eighth cervical vertebra. Be- 

 hind the eighth neural plate, three other mediajipygal plates 

 (Fig. 63, Py) succeed one another. The anterior two of 

 these are united by sutures with the eighth neural and 

 costals. and with one another ; but the thii-d is connected 

 externally only with the marginal plates. AH three are 

 perfectly distinct from the subjacent vertebra. 



The sides of the carapace are completed, between the 

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

 63. Mj on each side. Eight of these receive the ends of 

 the ribs of the second to the ninth dorsal vertebrae, in the 

 way already described. 



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

 marginal plates of the carapace are membrane bones, de- 

 veloped in the integument, quite independently of either 

 the vei-tebrEe or the ribs. But it appears that the neural 

 plates and the costal plates exist, as expansions of the 

 cartilages of the neural spines and ribs of the primitive 

 vertebrae, before ossification 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 skuU, where the 

 parietal and frontal bones stand in the same relation 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 Chelonia. The plastron is 

 wholly composed of membrane bones, which are developed 

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

 behind, the umbilicus of the foetus. The latter, at least, 

 therefore belong to the abdomen, and the plastron is a 

 thoracico-abdominal structxire. 



