l(j CRYPTODIRA. 



jilliud genera {Chtndo and Emi/s). The centra arc prococlous, 

 except in the Cholydridte and Platystornida) ; in Chelydra and 

 Macrochmmiis the two or three anterior are prococlous, the next 

 biconvex, the rest opisthoccelons ; in rlaf>/sternu)n the hist ten arc 

 again prococlous. Transverse processes or costoids are present on 

 most of the vertebra) and connected with the centrum and the 

 arch. Neural si)ines arc not developed. Chevron bones arc absent, 

 or vestigial, except in the ChclydridLC and Platysternida?, which 

 have them well developed, and mostly intervertebral, although per- 

 taining more to the posterior part of the centrum than to the 

 anterior part of that following. 



Skull. — The skull of all Cryptodira is comparatively very convex, 

 mostly so in the Chelonida) and terrestrial Testudinidic, least iu the 

 Chelydrida). The deeper the skull the larger the orbits ; however, 

 in the Chelonidai the orbits, which are enormous iu the young and 

 half-grown, are comparatively small iu old age. The orbit is com- 

 pletely encircled by four or five bones, viz. the maxillary, the prae- 

 frontal(the frontal), the postfrontal, and thejugal; in Phitijsternum 

 alone the jugal is cxcludec], and, the pra;frontal forming a suture 

 with the postfrontal, only three bones enter the orbit. The bony 

 external nostril is siiigle, bounded by the prasmaxillaries, which are 

 constantly distinct, the raaxillaries, and the praifrontals ; the froutals 

 and the parietals are also distinct. The latter bones send down a 

 prolongation to the pterygoid, from which it is usually separated by 

 a small scale-like bone, the homologue of the columella or ei)iptery- 

 goid. The postfrontals are large in most genera, small and forming 

 a narrow postorbital arch in most land Tortoises, especially in Pyxis, 

 iu which it is extremely slender ; this postorbital arch is compara- 

 tively broad in other Testudinida', and in the Dermatemydidte and 

 Cinosternida) ; and in the Chelydridse, Platysternidae, and Chelonidae 

 the postfrontal unites in a long suture with tlie parietal, the whole 

 or greater part of the temple being covered by a bony roof. 



The cranial arches of other " Monimostylicate " Keptiles, viz. 

 the postfronto-squamosal, the quadrato-jugal, and the paricto-squa- 

 mosal, arc all three represented only in the Chelonida^, in whicli the 

 temporal roof reaches its greatest development. In all other Cryp- 

 todira the parietal is widely separated from the squamosal. In 

 some genera (CJieh/dra, Macrodemmys, Platysternvm, Emys, Cliry- 

 sanys, BeUia, Mdacochmmys, Ocadia) the two lateral arches are 

 represented in the " zygomatic " arch, whilst in all others the 

 squamosal is separated from the postfrontal, so that the lower only 

 is represented ; in a few ( Geoemyda, Cistudo) there is no bonj- 

 temporal arch, and the quadralo-jugal is rudimentary or absent. In 

 IStaurotyjms, Cinosternum, and Plutysternum the maxillary forms a 

 suture with the qr.adrato-jugal. 



The sui)raoccipital forms a crest which is produced beyond a line 

 drawn between the jmsterior extremities of the squamosals. The 

 foramen magnum is deeper than broad and bounded by the supra- 

 occipital and the exoccipitals, and in Cltelone and Tludassocliehjs 



