o68 



CLASS REPTILIA. 



life, or completely obliterated. All those vertebrae in which the 

 costal articulation is on the centrum below the neuro-central suture 

 may be reckoned as cervical ; their number varying from about 2 1 

 to nearly 40. The centrum of the atlas is well developed, and there 

 is a wedge-shaped intercentrum between the latter and the skull. 

 The true cervicals are succeeded by a few vertebrae in which the 

 costal articulation is partly on the arch and partly on the centrum, 

 for which the name of pectorals has been proposed. The dorsal 

 vertebrae have the costal articulation placed entirely on the arch, 

 and generally forming an elongated transverse process. The caudal 

 vertebrae are always furnished with true ribs, and also with chevron- 

 bones, which may not be united below. The structure of the pec- 

 toral girdle is very remarkable, and has given rise to considerable 

 diversity of opinion. In all forms the coracoids meet in a median 

 symphysis, which may be short (fig. 987) or very long (fig. 988). 



Fig. 987. — Ventral aspect of the pectoral girdle of Nothosaurus mirabilis; from the Muschel- 

 kalk of Wurtemberg. Reduced, ost, Clavicle and interclavicles ; sc, Scapula ; gl, Glenoid 

 cavity ; cor, Coracoid. 



In the generalised Nothosaurus (fig. 987) the scapula has only a 

 small ventral portion, separated by a wide interval from that of its 

 fellow. Anteriorly to these ventral plates of the scapulae there 

 is a slender arch consisting of a median and two lateral por- 

 tions, corresponding to a similarly situated bone in Plesio- 

 saurus. This arch is usually correlated with the interclavicle and 

 clavicles ; but from the deep-seated position of its representative 

 in Plesiosaurus, Mr Hulke considers that in that genus it corre- 

 sponds to the omosternum of the Amphibia, and if this interpre- 



