ORDER CHELONIA. 1087 



costal bones (fig. 1009 bis) formed by the development of a plate on the 

 outer surface of each rib (fig. 1010); while the sides of the carapace 

 are completed by the eleven marginals?- which are dermal ossifica- 

 tions, and eight of which receive the extremities of the ribs from the 

 second to the ninth. In all young individuals, and in many of the 

 marine Turtles (fig. 1009 bis), the costal bones do not extend to the 

 extremities of the ribs, and consequently leave vacuities on the inner 

 side of the marginals, but in the land Tortoises and their allies (fig. 

 10 1 7) the carapace is entirely bony. In some instances, again, the 

 number of neural bones may be reduced (as in fig. 1014, where 

 there are but seven), and very rarely they are entirely wanting, so 

 that all the costals meet in the middle line. The two suprapygals 

 may also be reduced to one, as in fig. 1014. Similar variations 

 occur in respect to the degree of ossification of the plastron • since 



Fig. 1010. — Transverse section through the shell oiChelone mydas. Reduced, c, Vertebral 

 centrum ; n, Expanded neural spine ; r, Rib ; c', Costal bone ; m, Marginal do. ; /, Plastron. 

 (After Huxley.) 



while in all young individuals, and in the existing marine forms (fig. 

 1009) throughout a great part or the whole of life there are vacuities 

 between the bones, in the land Tortoises and their allies the bones 

 are all connected by suture. 



In the marine Turtles the plastron is totally unconnected with 

 the carapace ; but in most other forms the hyo- and hypoplastrals 

 send up longer or shorter peduncles underlying the marginals, and 

 in some cases also the costals of the carapace, and thus form well- 

 marked axillary and inguinal buttresses. These peduncles are 

 longest in some of the existing Pleurodira ; while the inward prom- 

 inence of the buttresses is most marked in the Indian Batagurs. 



Considerable variation occurs in the skulls of the different groups, 

 but it can only be mentioned here that in some genera like Chelydra 

 and Chelo?ie the supratemporal fossa is more or less completely 

 roofed over by the development of plates extending outwards from 

 the parietal and backwards from the postfrontal ; this roof being 



1 Occasionally the number of marginals may be increased to twelve or reduced 

 to ten. 



