124 



CARAPACE OF THE TURTLE. 



aquatic animals; and, being incapable of retraction 

 within the carapace, ossification extends in the direc- 

 tion of the fascia covering the temporal mnscles, 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, 

 11, postfrontals, 12, parietals, 7, and mastoids, 8. 



The carapace (Fig. 21) is composed of a series of median 

 and symmetrical pieces c/i, si to sll, and of two series 

 of unsymmetrical pieces on each side. The median pieces 

 have been regarded as lateral expansions of the summits 

 of the neural spines; the medio-lateral pieces as similar 

 developments of the ribs ; and the marginal pieces as the 

 homologues of the sternal ribs. But the development 

 of the carapace shows that ossification begins independ- 



Fig. 21. 



CARAPACE OF TURTLE {Chelone imbricata). 



ently in a fibro-cartilaginous matrix of the corium in the 

 first, cA, and some of the last, 5 9 to sll, median plates, 



