168 



Order CHELONIA. 



Family Marina. Turtles. 



Genus Chelone. 



769. The skeleton of the green Turtle (Chelone mydas). 



In the marine species of the Chelonian Order, of which this may be regarded as the type, 

 the ossification of the carapace and plastron is less extensive, and the whole skeleton is lighter, 

 than in those species that live on dry land. The head is proportionally larger, a character 

 common to aquatic animals ; and, being incapable of retraction within the carapace, ossifica- 

 tion extends in the direction of the fascia covering the temporal muscles, and forms a second 

 bony covering of the cranial cavity : this accessory defence is not due to the intercalation of 

 any new bones, but to exogenous growths from the frontals (n), postfrontals (12), parietals (7) 

 and mastoids (s). 



The carapace is composed of a series of median and symmetrical pieces, and of two series 

 of unsymmetrical pieces on each side. The median pieces have been regarded as lateral ex- 

 pansions of the summits of the neural spines * ; the medio-lateral pieces as similar develop- 

 ' ments of the ribsf ; and the marginal pieces as the homologues of the sternal ribs J. But 

 the development of the carapace shows that ossification begins independently in a fibro-carti- 

 laginous matrix of the corium in the first and some of the last median plates, and extends 

 from the summits of the neural spines into only eight of the intervening plates : ossification 

 also extends into the contiguous lateral plates, in some Chelonia, not from the corresponding 

 part of the subjacent ribs, but from points alternately nearer and farther from their heads, 

 showing that such extension of ossification into the corium is not a development of the 

 tubercle of the rib, as has been supposed. Ossification commences independently in the 

 corium in all the marginal plates which never coalesce with the bones uniting the sternum 

 with the vertebral ribs, and which are often more numerous, and sometimes less numerous 

 than those ribs, and in a few species are wanting. Whence it is to be inferred that the ex- 

 panded bones of the carapace, which supported and are impressed by the thick epidermal 

 scutes called ' tortoise shell,' are dermal ossifications, homologous with those which support 

 the nuchal and dorsal epidermal scutes in the Crocodile . Most of the pieces of the cara- 

 pace being directly continuous or connate with the obvious elements of the vertebrae, which 

 have been supposed exclusively to form them by their unusual expansion, the median ones 

 have been called ' neural plates,' and the medio-lateral pieces ' costal plates ' : but the ester- 



* CUVIER, Le9ons d' Anatomic Comparde, i. (1799) p. 212. f Ibid. p. 211. 



J GEOFFHOY, Annales du Muse'um, t. xiv. (1809) p. 7. 



$ CARUS, Lehrbuch der Vergleich. Anatomic, Bd. i. p. 164. PETERS, Observations ad Anato- 

 miam Cheloniorum. OWEN, Philos. Trans. 1849, p. 151. 



