ORDER CHELONIA. III7 



vical vertebrae have no distinct transverse processes, and the eighth 

 articulates with the first dorsal solely by the zygapophyses, thus 

 forming the most remarkably complete hinge-joint among the whole 

 of the Vertebrata. The fourth digit in each limb is characterised by 

 having not less than four phalangeals. The form of the tympanic 

 ring of the quadrate, and the relations of the latter to the mandible, 

 are of the same type as in the Cryptodira ; and we find a similar 

 absence of union between the pelvis and the plastron. In the 

 cranium (fig. 1022) the chief distinctive features are to be found in 

 the palate ; thus the broad pterygoids are separated from one another 

 by the basisphenoid, which extends forwards to join the palatines ; 

 the latter, owing to the small size of the vomer, uniting extensively 

 in the middle line ; and the whole structure of the palate being of a 

 Pleurodiran type. As minor characters of the skull, attention may 

 be directed to the open temporal fossae, and the forward position 

 and upward direction of the small orbits, as well as to the extreme 

 development of the backward processes of the supraoccipital and 

 squamosal. The sacral and caudal ribs articulate only with the 

 arches of the vertebrae, and there are no chevrons. The plastron is 

 always entirely separate from the carapace, and has a large median 

 vacuity and digitate extremities, as in the marine Chelonidce. Fur- 

 ther, the head and neck are retracted within the carapace after the 

 Cryptodiran manner, but, owing to the peculiar structure of the 

 eighth cervical vertebra, in a still more complete way. In all cases 

 the posterior lateral surfaces of the neural bones of the carapace are 

 much shorter than the anterior. 



The marked resemblance of the palate and the general aspect of 

 the Trionychoid skull to that of existing Pleurodira is a circum- 

 stance that does not admit of a ready explanation from a phylo- 

 genetic point of view, unless we adopt the somewhat improbable 

 suggestion that the Pleurodira gave origin to the Trionychoidea at a 

 time when their pterygoids had attained their full width, but while 

 the tympanic ring was still partially open. 



Family Trionychid^e. — The whole of the members of this sec- 

 tion may be included in a single family, which is, however, suscep- 

 tible of division into two subfamilies. In the first, or JEmydince, 

 the opisthotic of the skull unites with the pterygoid to divide the 

 posterior aperture of the auditory labyrinth into two foramina ; the 

 hyo- and hypoplastral of either side are fused together ; the sculp- 

 ture of the carapace is tuberculated ; and marginals may be present. 

 The existing Oriental genus Emyda, in which marginals and a com- 

 plete series of neurals are present, is represented by numerous species 

 in the Pliocene Siwaliks of India, some of which attained very large 

 dimensions. The African genera Cyclanorbis and Cycloderma, in 

 which marginals were not developed, are at present unknown in a 



