,, 



893.] Clavicular Arch in Ichtliyosauria and Sauropterygia. 165 



tion of an osteological separation of one bone into two bones which 

 has no existence. 



But the effect of the extraordinary difference in number of vertebrae 

 in the several regions of the body in Plesiosaurs and Chelonians 

 cannot, I think, be ignored in judging between the hypothesis of 

 morphological displacement of one limb of the scapula as against 

 morphological development of the other limb, when the bone is con- 

 sidered in Plesiosaurs from the point of view of a Chelonian com- 

 parison. The morphological resemblance with Chelonians is not so 

 close as to solve the problem by comparison only. 



Finally, there remains the clavicular arch, or, as Mr. Hulke terms 

 the bones, omosternalia. Mr. Hulke regards the interclavicle as 

 derived from the mesial ends of the clavicles (p. 246), and the 

 omosternum as derived from the epicoracoids, so that in existing 

 animals these structures appear to originate differently. Unfortu- 

 nately there are no epicoracoids preserved in the Sauropterygia, so 

 that it is impossible to base the nomenclature which Mr. Hulke 

 prefers upon a morphological or structural basis. 



The reason why the omosternal interpretation has been preferred 

 by Mr. Hulke is stated (p. 252) to be the undisputed deep position of 

 the bones, which are sometimes completely hidden by the scapulae, 

 and which always rest upon the visceral, as distinguished from the 

 ventral, surface of those bones ; and this, coupled with the composite 

 structure, is the only reason advanced. I would compare this 

 nomenclature for the bones, which I have figured as clavicular 

 (' Roy. Soc. Proc.,' vol. 51, pp. 129, 131, 133, 140, 147, <fco.), with the 

 nomenclature adopted by Mr. Hulke for the corresponding bones in 

 Nothosaurus ('Proc. Roy. Soc.,' vol. 52, p. 240). The interclavicle 

 and clavicles are there represented, and the clavicles are correctly 

 shown to extend on the deep-seated or visceral surface of what I 

 regard as the scapulae, precisely as in the Sauropterygia. So that 

 the supposed proof from deep-seated position, which shows the bone 

 not to be clavicle but omosternum in Plesiosaurus is exactly the same 

 as that which is considered by Mr. Hulke to 'prove the bones which 

 correspond to them in position in Nothosaurus, to be not omosternal. 

 but clavicular. In both types the scapula is placed horizontally in 

 advance of the coracoid, although there are some differences of form 

 in the bones as compared with the Plesiosaurian genera. But all 

 von Meyer's specimens show that the clavicles in Nothosaurus extend 

 upon the visceral surfaces of the scapulae, precisely as in Plesiosaurus. 

 Mr. Hulke does not question that the bone named scapula in that 

 genus represents the scapula. There is no suggestion that it includes 

 the precoracoid ; and it appears to be suggested that the part of the 

 coracoid which is internal to the notch in the anterior part of that 

 bone is precoracoid ; so that the precoracoid would be an indivisible 



