and Clavicular Arch in Sauropterygia. 



12] 



these three bones terminates in a sharp edge or point, and no one of 

 them shows a terminal surface which has the aspect of being carti- 

 laginous ; there is therefore no evidence of a cartilaginous origin, 

 while the mutual relations of the bones and their forms closely- 

 parallel the clavicles in Chelonians and some Lacertilians ; they are 

 therefore identified with those elements,* because there is no ground 

 for regarding them as parts of the sternum. 



§ 2. In Sauropterygia. 



The nomenclature of the bones of the shoulder girdle in the Sauro- 

 pterygia has given rise to greater differences of opinion.f The 

 Amphibian mode of ossification of the long bones by long conical 

 epiphyses^ connected by a dice-box shaped shaft, coupled with a 

 general resemblance of the pelvis and shoulder girdle to those of 

 Chelonians, led to the conclusion that, while the scapula, precoracoid, 

 and coracoidare all separate from each other in some existing Amphi- 

 bians, the Amphibian plan had been modified in the Chelonia by the 

 scapula and precoracoid remaining undivided, while in the Sauro- 

 pterygia the coracoid and precoracoid remained in primitive undivided 

 condition. The strongest argument against this is the absence 

 of a recognisable coracoid foramen in Sauropterygia in the cora- 

 coid bone,§ which, when present, may be supposed always to occur 

 at the junction of the coracoid and precoracoid elements. At the 

 same time the clavicles were recognised in the Sauropterygia as 

 distinct lateral ossifications, placed at the sides of the interclavicle, 

 and closely united with it by suture in some species of Plesiosaurus. 



The difficulty with the scapular arch is in the theoretical pos- 

 tulates concerning the elements of which its several parts consist. 

 Sir R. Owen (' Brit. Assoc. Rep.,' 1839, p. 56) used the Chelonian 

 hypothesis to explain the scapula in Plesiosaurus. And E. D. Cope 

 ('Amer. Phil. Soc. Trans.,' 1870, p. 51), in Elasmosaurus, regarded 

 the ventral plate of the bone anterior to the coracoid as clavicles or 

 procoracoids, assuming a scapula external to it, as in Chelonians. And 

 in 1883 Mr. Hulke (loc. cit.) also urged that the Plesiosaurian bone, 

 usually named scapula, is a compound bone formed of scapula and 

 precoracoid, as in Chelonians. 



In support of this interpretation, it might be urged that the bones 



* Hulke, loc. cit. 



f ' Those which prevailed prior to 1874 were summarised in the ' Quart. Journ 

 Greol. Soc.,' for that year. 



t « Index to Fossil Eept. Woodw. Mus.,' pp. 98, 120, 1869. 



§ The specimen figured ' Quart. Journ. Q-eol. Soc.,' 1874, p. 446, is suggestive, 

 but not conclusive, for the bone is very thin, and no portion of the margin of the 

 perforation is seen, so that its existence as a normal character is unproved. 



K 2 



