150 Prof. H. G. Seeley. The Shoulder Girdle 



The short-necked genera are distinguished as a group by having the 

 two articular costal facets placed chiefly at the sides of the centrum, 

 and not at its infero-lateral angle. This circumstance appears to 

 indicate that the neck is elongated chiefly by the addition of vertebrae 

 to its anterior portion. The mesopodial bones are two in number, and 

 more or less quadrate, with a tendency to transverse extension in the 

 ulna. The dorsal vertebrae are relatively long compared with those of 

 the neck. In Bhomaleosaurus the average length of the cervical centrum 

 is 2*66 inches, while the average length of the dorsal vertebrae is 

 3'2 inches. In Rhomaleosaurus the facets remain separated to the 

 end of the series, but in some species of Pliosaurus there is an 

 approximation of the facets in the posterior cervical vertebrae which 

 is not seen further forward. The relatively small size of the head in 

 Rhomaleosaurus as compared with Pliosaurus shows that the head is 

 not necessarily long in all the short-necked genera. The shoulder 

 girdle in the Rhomaleosaur being unknown at present, there is no 

 means of comparing it with that attributed to Pliosaurus. 



The long-necked genera have the cervical vertebrae in greater 

 number, and relatively longer, and, except the earliest, they are usually 

 as long as the dorsal vertebrae. The articulations for the cervical ribs 

 are elongated from front to back, longitudinally divided, but usually 

 so compressed from above downward that the division is only a narrow 

 shallow channel, always placed at the infero-posterior angle of the 

 centrum. The two facets are obvious in species like P. dolichodeirus ; 

 in many others they are only to be recognised by careful examination. 

 In this genus the radius is elongated, with its lateral borders concave 

 and ossified, and the distal end narrower than the proximal end. 



The scapulae and coracoids never meet in the median line, unless in 

 the genus Breimosaurus. This condition has been figured in the British 

 Museum specimen 2041 (' Greol. Soc. Quart. Journ.,' 1871, p. 446), 

 where a wide interspace is left between the coracoids and scapulae in 

 the median line in front, and there is a roughness upon the scapula as 

 though the interclavicle or clavicle had extended upon it. The inter- 

 clavicle usually completes the inner border of the coracoid foramen 

 in Plesiosaurus ; but in this fossil the relations of the bones are like 

 those attributed to Pliosaurus by Sir R. Owen. According to Mr. 

 Lydekker, what I regard as the pre-articular part of the scapula is 

 the humerus of P. HaivMusi ( k Cat. Foss. Rept. Brit. Mus.,' Part II, 

 p. 277). This is a matter that may be definitely determined by ex- 

 amination of the specimen, which comprises the scapula only, closely 

 united by suture to the coracoid. 



In the second division of the Sauropterygia or Elasmosauridae the 

 cervical ribs articulate by a single head with the centrum, the 

 scapulae, as well as the coracoids, meet each other in the median line, 

 the clavicles, so tar as they are known, are usually slender, the meso- 



