238 TKIONTCHOIDEA. 



to be looked upon as vestiges of the marginals of other Chelouians, 

 but have evidently an independent origin, like another similar 

 ossification, anterior to the nuchal, which is found in Emyda and 

 Cyclanorhis. 



Plastron, — The plastron forms, as in the Marine Turtles and the 

 young of all Chelonians, an incomplete shield, a median vacuity 

 being present throughout life between the lateral elements. These, 

 the hyo- and hypoplastra, are joined by suture, or may even (in 

 Cijcloderma, Em>/da, and Cyclanorhis) be fused into a single bone 

 as early as birth, although distinct in the embryo * ; but their inner 

 and outer borders form deep dentations, and there is no connection, 

 with the dorsal shield. The xiphiplastra join each other and the 

 hypoplastra by interlocked digitations. The three other elements, 

 viz. the epiplastra (clavicles) and the entoplastron (interclavicle), 

 differ greatly from theii- homologues in other Chelonians, each of 

 these being angular or chevron-shaped, the three forming together 

 a A- oi' X-shaped figure ; the posterior branch of each epiplastron is 

 in contact with the entoplastron, and separated by the extremity of 

 the latter from the hyoplastron. Some or all of the plastral bones 

 develop with age superficial rugose or sculptured plates, the so- 

 called plastral callosities, which may extend considerably beyond 

 the limits of, and differ greatly in shape from, the original bones. 

 In Trionyx these plates are constantly jjresent on the hyo-hypo- 

 plastra, usually on the xiphiplastra, and more rarely, and as a rule 

 only in fully adult specimens, on the entoplastron ; in Cydoderma, 

 Emyda, and Cyclanorhis, each bone has its sculptured plate, and in 

 the latter genus there are a pair of additional similar plates in 

 front of the epiplastra and often other, smaller, independent ossifi- 

 cations as well. 



Cervical Yertebe^. — In these essentially "Cryptodiran" Turtles 

 the neck is more perfectly adapted for complete and rapid retraction 

 than in any other Chelonian t. The cervical region at least equals, 

 and usually exceeds, the length of the dorsal vertebral column. The 

 most remarkable peculiarity resides in the structure of the last or 

 eighth vertebra, the centrum of which terminates in a thin plate and 

 does not articulate with the fij'st dorsal ; the articulation being 

 merely by means of the zygapophyses, in such a way that the two 

 vertebrae form together an angle when the neck is stretched out, 

 and are applied closely by their lower surfaces when the neck is 

 retracted. 



The four elements of the atlas are distinct, viz. the neural arches, the 

 hypapophysis (pseudocentrum), and the centrum (odontoid process). 

 The following vertebra) are much elongate, compressed in the middle, 

 and transverse processes are absent, or merely represented on the 



* Anderson has found the hyoplastron formed of two bones, an outer and 

 an inner, in an embryo oi Emyda scufata. J. Linn. Soc. xii. 1870, p. 514. 



t Anderson (Anat. Zool. Ees. Yunnan, p. 78.5) rcmarijs that the neek when 

 retracted is so doubled on itself, that the base of the cervical vertebrix;, at the 

 anterior extremity of the carapace, is on the same line with the tip of the snout, 

 and the posterior bend opposite the inguinal notch of the plastron. 



