4 CHELONIA. 



forming an incomplete series at the posterior extremity of the shell, 

 and having no connection with the ribs. Skull with the mandibular 

 articulation and the notched tympanic ring of the Cryptodiran type 1 ; 

 the broad pterygoids separated in the median line by the basi- 

 sphenoid, which joins the palatines anteriorly ; the latter meeting 

 in a median suture, owing to the small size of the vomer. Cervical 

 vertebras elongated, without distinct transverse processes ; the 

 eighth articulating to the first dorsal solely by the zygapophyses. 

 Sacral and caudal ribs articulating with the arches of the vertebras ; 

 no chevrons. Pour or more phalangeals in the fourth digit. Head 

 retracted after the Cryptodiran manner l . 



Family TRIONYCHIDiE. 



Skull depressed, with the small orbits directed more or less 

 upwards and approximated towards the nares ; the temporal fossae 

 completely open, and the squamosal and supraoccipital with very- 

 long posterior processes. Tympanic cavity completely closed, with 

 the exception of the small aperture for the stapes. Neural bones 

 of carapace with the posterior lateral facet much shorter than the 

 anterior ; plastron totally distinct from carapace, with large vacui- 

 ties. Angle formed by junction of scapula and precoracoid acute. 

 Humerus much curved, being of the same general structure as in 

 the Chelydridce (fig. 32), but with a larger and narrower head. 



All the known forms are referable to this family. 



Subfamily Trionychin^:. 



Hyoplastral distinct from hypoplastral ; outer extremities of 

 nuchal bone overlying rib supporting first costal bone 2 ; pterygoid 

 not uniting with opisthotic to divide posterior aperture of labyrinth ; 

 no marginals ; sculpture generally vermiculate, or pitted ; neurals 

 always well developed. 



Genus CHITRA, Gray 3 . 



Skull long and narrow, with the orbits placed at the anterior 

 extremity, and the postorbital bar wider than twice the diameter of 

 the orbit. Eight neurals. Hypoplastral with three digitations at 

 postero-external angle. Humerus with ectepi condylar groove 

 forming a long channel on preaxial border of shaft. 



1 Vide infra. 



- The rib supporting the first costal bone is the second . 



3 Cat. Tortoises Brit. Mus. p. 49 (1844). 



