1076 



CLASS REPTILIA. 



than the others in the more typical species. The neck is more or 

 less elongated, with the anterior vertebrae in most cases very small. 



The cervical ribs and neu- 

 ral arches are firmly articu- 

 lated to the centra of the 

 vertebrae, but traces of the 

 suture usually persist ; in the 

 cervical region the facets for 

 the articulation of the ribs 

 are usually double, and the 

 terminal faces of the centra 

 generally ellipsoidal, and 

 more or less cupped. The 

 most characteristic features 

 of the genus are, however, 

 to be found in the pectoral 

 girdle (fig. 997), in which 

 the scapulae are relatively 

 small, and widely separated 

 in the middle line, where 

 they rest upon the inter- 

 clavicle, with its small and 

 deep notch. The coracoids 

 are, moreover, long and rather narrow, with a median production 

 in advance of the glenoid cavity ; while the foramen between 

 the coracoid and scapula is very large and open towards the 

 interclavicle. 



Fig. 997. — Ventral aspect ot the pectoral girdle ot 

 Plesiosaurus dolichodirus ; from the Lower Lias of 

 Dorsetshire. Reduced, ost, Interclavicle; sc, Scap- 

 ula ; pc, Ventral plate of do. ; co, Coracoid ; H, Head 

 of humerus. (After Hulke.) 



This genus may be divided into three groups. The Longirostrine 

 group, represented by P. rostratus of the Lower and P. lo?igirostris of 

 the Upper Lias, is characterised by the comparatively elongated man- 

 dibular symphysis, and the extremely short neural spines and single 

 costal facets of the majority of the cervical vertebrae. In the typical 

 group we have P. dolichodinis with its extremely long neck (fig. 995), the 

 shorter-necked P. Hawkinsi, and the large P. Conybeari, all being from 

 the Lower Lias. These forms have a moderately short mandibular sym- 

 physis (fig. 996, c), while the centra of the cervical vertebrae are never 

 greatly elongated, and there are double costal facets and moderately tall 

 neural spines in this part of the vertebral column. The third group is 

 represented only by P. homalospo?idylus, of the Upper Lias, which is 

 characterised by the great elongation of the centra of the cervical ver- 

 tebrae, which have flat terminal faces, and enormously tall neural spines. 

 The resemblance of these vertebrae to those of the cervicals of the typical 

 group of the Jurassic and Cretaceous genus Cimoliosaurus suggests the 

 origin of the latter group from the present ; whereas on similar grounds 

 it may be suggested that the Ccelospondyline group of Cimoliosaurus has 

 originated from the typical group of Plesiosaurus. In all species of the 

 present genus the radius and ulna (fig. 994) still retain evidence of their 



