162 Prof. H. G. Seeley. The Shoulder Girdle and [June 11 



which diverge so as to include between them a large angle, 

 ventral ray extends transversely inward to the median ventral Hi 

 towards its fellow, in a way to which no part of the bone in the 

 genus Plesiosaurus offers a parallel ; the dorsal ray ascends to 

 carapace in a way which is equally unparalleled in Plesiosai 

 There are two methods in which these structures may be compai 

 in the two groups. 



First, we may suppose, as Mr. Hulke does, that although the coi 

 coids have no median union with each other in Chelonians, they 

 strictly comparable with the Plesiosaurian coracoids, which unite 

 a median suture. Then the ray of the anterior bone, instead of 

 tending inward to meet its fellow, as in Chelonia,ns, may be suppose 

 to be directed forward to become an expanded plate, uniting with 

 its fellow in some genera of Sauropterygia (e.g., Murcenosaurus) in 

 the median line, and in such cases it may send a ray back to make 

 a median union with the coracoid. Further, the vertical or for- 

 wardly inclined bar of the Chelonian bone is supposed in Plesio- 

 saurs to be represented by the compressed plate which, ascending 

 from the horizontal ray, extends above the articular surface for the 

 bumerus. Hence the two rays of the bone in Plesiosaurs are in two 

 planes, one horizontal, and the other vertical, while in Chelonians 

 the two rays may be regarded as substantially in the same plane, 

 which, in so far as it is not vertical, is inclined forward. In both 

 groups of animals Mr. Hulke names the ventral ray precoracoid, and 

 the dorsal ray scapula. 



There is another way of bringing the two types of shoulder girdle into 

 comparison. If the elongation of the neck in Plesiosaurus is supposed 

 to be brought about by an augmentation in number of the vertebraa 

 from a type in which they were less numerous, then there is some 

 ground for anticipating that the scapula, if originally in such a 

 vertical position as it holds in Chelonians, would have its free supe- 

 rior end carried forward, until it might come to be in the same hori- 

 zontal plane with the coracoid.* This is what I infer to have 

 happened, and to represent the mutual relations of the parts, so that 

 the forwardly directed horizontal plate of bone in Plesiosaurus would 

 be homologous with the vertically directed bar of bone in Chelonians 

 which all anatomists agree in naming scapula. And, therefore, 

 there would be nothing in the Plesiosaurian shoulder girdle to cor- 

 respond to the ray of bone which is directed inward ventrally in 

 Chelonians, and that element, I suppose, to have practically disap- 

 peared from the Plesiosaurian skeleton. Hence the ascending ray of 

 the scapula in Plesiosaurus, which extends above the humeral articu- 

 lation, would not be homologous with the vertical ray of the scapula 



* Some generic modifications of the Plesiosaurian pectoral arch, ' Quart. Jour. 

 Geol. Soc.,' vol. 30, 1874, p. 439. 



