CHELONIA. 6 



crest of the Crocodilian humerus ; while the ulnar process (i) 

 represents the inner tuberosity of the same. Among the Testudi- 

 nata the radial process in all Pleurodira (fig. 1, B) is comparatively 

 small, while the ulnar process is placed in the same transverse line 

 as the distal surface of the bone. In many existing Cryptodira 

 (fig. 1, C), and more especially the Land-Tortoises, the radial process 

 is developed into a thin plate directed towards the ventral aspect of 

 the bone ; while the ulnar process is likewise twisted round to the 

 same side. In the Cheloniclce the radial process tends to become 

 aborted, and to attain a position more or less below the level of the 

 head. In the Athecata, which likewise have a straight humerus, the 

 radial process, while descending on the shaft, tends, on the other 

 hand, to attain an excessive development. 



The terms employed in the description of the Chelonian shell are 

 those used in Boulenger's ' Catalogue of Chelonians, &c.' 1 The 

 bones placed between the marginal pygal and the last neural are, 

 however, termed suprapygals ; and the adjectival termination is 

 applied to the bony elements of the plastron. The horny shields 

 are likewise named after the same manner 2 , with the exception 

 that the term caudal is used in place of supracaudal. To obviate 

 the chance of confusion between the sutures of bones and the lines 

 of junction of the horny shields the term sulcus has been applied to 

 the depressions on the shell formed by the latter. 



Suborder I. TESTUDINATA. 



Carapace mesially composed of expansions of the ribs, and usually 

 also of the dorsal vertebrae, to both of which it is immovably welded. 

 Skull with the parietals giving off vertically descending plates, 

 which unite, either directly, or by the intervention of the columellas 

 (epipterygoids), with the pterygoids. 



The name Testudinata is employed in preference to that of 

 Thecophora, since the latter term (Thecaphora) is used for a group 

 of Hydroid Zoophyta. 



Section I. TRIONYCHOIDEA. 



Shell free from pelvis, and (at least in existing forms) sculptured 

 and devoid of epidermal shields ; entoplastral chevron-shaped and 

 separating the epi- and hyoplastrals ; marginals 3 (when present) 



1 Pages 11-15. 2 Ibid. p. 19, fig. 2. 



3 The marginals of the Trionychoidea are regarded by Boulenger as not 

 homologous with those of other Testudinata. 



b2 



