ORDER VI 



CHELONIA 



193 



The postfrontals are large in most Cryptodires, forming a postorbital 

 arch ; in the marine turtles the postfrontal unites in a long suture with the 

 parietal, the whole or greater part of the temporal region being roofed over by 

 bone. The temporal roof attains its greatest development in the Chelonidae, 

 where three cranial arches are present — the postfronto-squamosal, the jugo- 

 quadratojugal, and the parieto- squamosal. In all other Cryptodires the 

 parietal is widely separated from the squamosal, and as a rule the squamosal 

 is separated from the postfrontal. In a few genera there is no bony temporal 

 arch, and the quadra to-jugal is rudimentary or absent. The jugal, when 

 present, takes part with the maxilla in the lower border of the orbit, being 

 excluded from it only in the genus Platy sternum. The prootic and opisthotic 



N 



B 

 Pmx 



Prf / Aj ^ > \ 



M * M14$^ 



Q.uJ 



tin J 



Fig. 298. 



Trionyx gangetieus, Cuvier. Recent; India. Superior (.4), and palatal (B) aspects of skull, reduced, 

 bo, Basioccipital ; bsph, Basisphenoid ; ch, Internal nares ; exo, Exoccipital ; fir, Frontal ; j, Jugal ; mx, Maxilla ; 

 N, External nostril ; op, Opisthotic ; pa, Parietal ; pi, Palatine ; pmx, Premaxilla ; prf, Prefrontal + nasal ; pro, 

 Prootic ; ptf, Postfrontal ; q, Quadrate ; qnj, Quadrato-jugal ; S, Supratemporal fossa ; so, Supraoccipital ; sq, 

 Squamosal ; vo, Vomer. 



are both stout plates, situated in their usual positions. The exoccipital 

 sometimes fuses with the supraoccipital, which is produced beyond a line 

 drawn between the posterior extremities of the squamosal. The foramen 

 magnum is deeper than broad, and bounded by the supraoccipital and the 

 exoccipitals, and occasionally also by the basioccipital. 



The squamosal joins both the prootic and opisthotic, and is buttressed by 

 the quadrate, which penetrates the otic region by means of a superiorly or 

 inwardly directed process, and is snturally united with the quadrato-jugal. 

 The quadrate sometimes joins both the basisphenoid and basioccipital, but in 

 all Cryptodires it is separated from the basisphenoid by the pterygoids, which 

 form a suture with the basioccipital, or very nearly reach the latter bone, and 

 are in contact with the maxillae (except in the Chelouidae). In none of the 

 Pleurodires do the pterygoids extend posteriorly beyond the quadrate. 

 VOL. II 



