1122 



CLASS REPTILIA. 



cylindrical and deeply fluted (fig. 1032), but are occasionally com- 

 pressed, carinated, and smooth (fig. 1028). The vertebral column 

 is primarily divisible only into a precaudal and a caudal series, 

 there being no sacrum. The centra (fig. 1025) are amphiccelous, 

 more or less disk-like, and either deeply cupped or nearly flat. On 

 either side those of the precaudal series (fig. 1025) carry a pair of 

 tubercles for the articulation of the double-headed ribs; while in the 

 caudal region these tubercles coalesce. Superiorly each centrum has a 

 pair of flattened surfaces for the attachment of the neural arches, 

 which are united merely by synchondrosis. In the cervical region 

 the tubercles for the ribs are placed near the summit of the lateral 

 surfaces of the centra, and they gradually descend on the centrum, 

 till they reach the base of the lateral surface in the posterior pre- 

 caudal and caudal regions. In some forms the upper costal tubercle 

 of the anterior vertebrae is placed either partly or entirely on the 

 neural arch, and it is convenient to term such vertebrae, or their 

 homologues, cervicals, and the remainder of the precaudal series 

 dorsals. The centrum of the atlas vertebra is well developed, and 



Fig. 1026. — A, Ventral aspect of the pectoral girdle and right limb of Ichthyosaurus inter- 

 medins. B, Pelvic girdle of do. Reduced, a, Interclavicle ; b, Clavicle ; c, Scapula ; d, 

 Coracoid ; e, Humerus ; f, Ulna ; g, Radius ; it, Ilium ; is, Ischium ; ft, Pubis. (After 

 Huxley.) 



there is a large intercentum between the atlas and the skull, and 

 another between the former and the axis. There was no sternum ; 

 but a complex system of abdominal ribs was developed. The 

 dorsal ribs were devoid of uncinate processes ; and ribs occur in 

 the caudal region. In the pectoral girdle (fig. 1026, a) there is 

 no precoracoid ; but clavicles and a T-shaped interclavicle, like the 

 corresponding bones of the Lacertilia, were developed. The cora- 

 coids were large and expanded, devoid of any fontanelle, and not 



