16 CRYPTODIRA. 
allied genera (Cistudo and Emys). The centra are proccelous, 
except in the Chelydride and Platysternide; in Chelydra and 
Macroclemmys the two or three anterior are proccelous, the next 
biconvex, the rest opisthoccelous ; in Platysternum the last ten are 
again proceelous. ‘Transverse processes or costoids are present on 
most of the vertebree and connected with the centrum and the 
arch. Neural spines are not developed. Chevron bones are absent, 
or vestigial, except in the Chelydride and Platysternide, 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. 
Sxutt.—The skull of all Or ppbodiealt is comparatively very convex, 
mostly so in the Chelonide and terrestrial Testudinidiw, least in the 
Chelydridee. The deeper the skull the larger the orbits ; however, 
in the Chelonide the orbits, which are enormous in the young and 
half-grown, are comparatively small in old age. The orbit is com- 
pletely anvils by four or five bones, viz. fie maxillary, the pree- 
frontal (the frontal), the postfrontal, and the jugal; in Platysternum 
alone the jugal is excluded, and, the prefrontal forming a suture 
with the postfrontal, only three bones enter the orbit. ‘The bony 
external nostril is single, bounded by the premaxillaries, which are 
constantly distinct, the maxillaries, and the preefrontals ; the frontals 
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 epiptery- 
sod. he postfrontals are large in most genera, small and forming 
a narrow postorbital arch in most land Tortoises, especially in Pywis, 
in which it is extremely slender; this postorbital arch is compara- 
tively broad in other Testudinide, and in the Dermatemydide and 
Cinosternide ; and in the Chelydride, Platysternidee, and Chelonidse 
the postfrontal unites in a long suture with the parietal, the whole 
or greater part of the temple being covered by a bony roof. 
The cranial arches of other ‘* Monimostylicate” Reptiles, viz. 
the postfronto-squamosal, the quadrato-jugal, and the parieto-squa- 
mosal, are all three represented only in the Chelonide, in which the 
temporal roof reaches its greatest development. In all other Cryp- 
todira the parietal is widely separated from the squamosal. In 
some genera (Chelydra, Macroclemmys, Platysternum, Emys, Chry- 
semys, Bellia, Malacoclemmys, 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 bony 
temporal arch, and the quadrato-jugal is rudimentary or absent. In 
Staurotypus, Cinosternum, and Platysternum the maxillary forms a 
suture with the quadrato-jugal. 
The supraoccipital forms a crest which is produced beyond a line 
drawn between the posterior extremities of the squamosals. The 
foramen magnum is deeper than broad and bounded by the supra- 
occipital and the exoccipitals, and in Chelone and Thalassochelys 
