G4 ANATOMY OF VERTEBRATES. 



sternal ribs, connate in a more or less complete degree with dermal 

 bony plates. There were five pairs in the extinct Pleurosternon. 



In the marine Chelonians the dermal ossifications, fig. 52, pi 1 to 

 8, do not cover the whole of the intercostal spaces ; the slender 

 ribs project beyond them. In the fresh-water and land kinds 

 they extend to the marginal plates and complete the bony roof, as 

 in fio\ 51. There is a similar difference in the degree of ossification 

 of the e plastron ' between the genus Chclone and the genera Emys 

 and Testudo. 



In the Chelonia the true centrum of the atlas does not coalesce, 

 as an 'odontoid' process, with that of the axis, and usually supports 

 its own neural arch: the hypapophysis is proportionally reduced. 1 

 All the eight cervical vertebra?, fig. 51, e, are free, movable, and 

 ribless : the fourth of these vertebra? has a much elongated centrum, 

 which is convex at both ends : the eighth is short and broad, with 

 the anterior surface of the body divided into two transversely 

 elongated convexities, and the posterior part of the body forming 

 a single convex surface divided into two lateral facets ; the under 

 part of the centrum is carinate ; the neural arch, which is 

 anchylosed to this centrum, is short, broad, obtuse, and overarched 

 by the broad expanded nuchal plate. The first dorsal vertebra 

 is also short and broad, with two short and thick pleurapophyses, 

 articulated by one end to the expanded anterior part of the 

 centrum, and united by suture at the other end to the succeeding 

 pair of ribs. The head of each rib of the second pair is supported 

 upon a strong trihedral neck, and articulated to the interspace of 

 the first and second dorsal vertebra? : it is connate, at the part 

 corresponding to the tubercle, with the first broad costal plate, 

 which articulates by suture to the lateral margin of the first 

 neural plate, and to portions of the nuchal and third neural 

 plates : the connate rib, which is almost lost in the substance of 

 the costal plate, is continued with it to the anterior and outer part 

 of the carapace, where it resumes its subcylindrical form, and 

 articulates with the second and third marginal pieces of the cara- 

 pace. The neural arch of the second dorsal vertebra is shifted 

 forwards to the interspace between its own centrum and that of 

 the first dorsal vertebra. A similar disposition of the neural 

 arch and of the ribs prevails in the third to the ninth dorsal 

 vertebra? inclusive. The bony floor of the great abdominal box, 

 or ' plastron,' is formed by the ha?mapophyses and sternum connate 

 with dermal osseous plates, forming, as in the turtle, nine pieces, 



1 clxvii. p. 435, pi. xiii. 



