1893.] Clavicular Arch in Ichthyosauria and Sauropterugia. 167 



[There is another point in Mr. Hulke's argument. He adopts the 

 classification of the shoulder girdle bones into primary or cartilaginous 

 and secondary or membranous. There is no doubt that in the im- 

 mature Plesiosaurus, in which all the indubitable cartilage bones show 

 unossified cartilaginous surfaces and margins, the two bones figured 

 (' Roy. Soc. Proc.,' vol. 51, p. 133) which I regard as clavicles are com- 

 pletely ossified, with sharp well-defined margins, and show no signs 

 of immaturity ; and I therefore regarded them as membrane bones. 

 Some of the clavicular bones figured at the same time (loc. cit., p. 131) 

 are almost as thin as bones could be, and in marked contrast to the 

 cartilage bones of the skeletons with which they are severally 

 associated, although there are other examples in the collection of 

 Mr. A. N. Leeds, of which he has since had the kindness to send me, 

 drawings, which are considerably thicker. If these bones had been 

 omosternal bones, segmented from epicoracoids which are cartilages, 

 presumably they would have been cartilage bones. But the immature 

 specimens to which I have referred show no indication of having had 

 a cartilaginous origin. Mr. Hulke states that it is possible that the 

 bones which he terms omosternal are membrane bones, but adds that 

 this is not yet absolutely certain, and yet, having urged that the dis- 

 tinction between the two groups of bones rests upon their different 

 origin in the embryo, concludes that the weight of evidence is still in 

 favour of an omosternal homology. 



I am unable to imagine any evidence more conclusive than that 

 which has been brought forward, based upon the condition of the 

 bones themselves in the young Plesiosaur, and comparison with the 

 condition in Nothosaurus. 



From 1874 (* Quart. Jour. Greol. Soc/), I have indicated affinities 

 between Sauropterygia and other animals. The comparisons which 

 have weight are indicative of a common plan between the animal 

 types compared. Affinities which can thus be demonstrated may 

 justify views of homology in interpreting obscure parts of the 

 skeleton, which are more valuable than the views based upon 

 resemblances of form found in isolated bones in animals which 

 are widely different in organisation. I have formerly pointed out 

 elements of the skeleton in which Plesiosaurs show characteristics of 

 Amphibians, as in the mode of ossification of their long bones. 

 Exactly the same condition of ossification is found in Nothosaurus. 

 If this is an Amphibian inheritance which amounts to identity of plan 

 iu the construction of the limb bones, in a way which marks these 

 two orders of animals off from other groups, it does not furnish an 

 a. priori ground for assuming that the coracoid, the scapula, and the 

 arch of bones in front of these are all morphologically different in 

 the Plesiosaurs and Nothosaurs, but rather that they are substantially 

 the same. 



