166 Prof. H. G. Seeley. The Shoulder Girdle and [June 15, 



portion of the coracoid. Hence, apparently, the difference in inter- 

 pretation, the omosternum being assumed to join the precoracoid 

 when it is present, while the clavicle is assumed to join the scapula. 

 This interpretation is entirely hypothetical ; for there is no more 

 evidence in favour of the Nothosaurian coracoid being a coraco- 

 precoracoid than there is in favour of the Plesiosaurian scapula being 

 a precoraco-scapula. And therefore there is no foundation for the 

 difference of interpretation which would name the bone which rests 

 upon the visceral surface of the anterior element of the shoulder 

 girdle in Nothosaurus a clavicle, and that which rests upon the 

 visceral surface of the bone which occupies the same position in 

 Plesiosaurus an omosternal bone. This arch in Nothosaurus consists 

 of a small median piece and two long lateral pieces, which may, I 

 think, be compared with the elements figured in Plesiosaurus ('Hoy. 

 Soc. Proc.,' vol. 51, p. 129), although the median element is much 

 larger in the Plesiosaur, and the lateral elements are much shorter. 

 Until some evidence is forthcoming to show that the boues which 

 form an arch in the same position and are similarly situate on the 

 visceral surface of the bones of the shoulder girdle are different, there 

 is no justification for applying different names to them, or assuming 

 that they are not homologous. In some of the smaller Nothosaurs 

 figured by von Meyer it is evident that the anterior visceral surface 

 of the scapula was smooth, so that the clavicle joined the scapula by 

 squamose overlap, and not by suture, as in the genus Nothosaurus, 

 thus approximating more nearly to the condition in Plesiosaurus. 



The circumstance that the interclavicle in Nothosaurus and its allies 

 is not wedged in behind the visceral surface of the coracoids, is 

 necessarily a consequence of the small antero-posterior development 

 of the median union between the coracoid bones, and the great length 

 of the clavicles, by which the interclavicle is carried forward in the 

 middle of an arch which is convex in front ; while in Plesiosaurs 

 what I believe to be the homologous arch is shorter and similarly 

 directed backward. But there is no difference in plan. In Plesio- 

 saurs (loc. cit., p. 129) the clavicles are directed backward exactly as 

 in Nothosaurus, and it is only the anterior margin of the inter- 

 clavicle which is concave in front. There is so much in common 

 in the structure of the skull, in the vertebral column, in the pelvic 

 arch, the shoulder girdle, and limbs between the Sauropterygia and 

 Nothosauria that there are probably no two other well differentiated 

 orders of animals which have greater organic affinity with each other, 

 and, therefore, although I have proposed to recognise a cartilaginous 

 precoracoid in Nothosaurus of which no evidence is available in 

 Plesiosaurus, I can see no ground for supposing that the bones of the 

 shoulder girdle which are actually preserved are not severally the 

 same as in the Sauropterygia. 



