Apr. 1903. North American Plesiosaurs— Willis ion. 47 



cation of this cartilage evidently occurs in some forms, uniting the 

 whole into a single ventral plate, which is said to have a sutural con- 

 nection with the epicoracoid processes, forming what Seeley calls the 

 Elasmosaurian type of girdle. Seeley suggests that this cartilage 

 may represent the precoracoid: "In all Plesiosaurs, on the other 

 hand, the precoracoid, if developed, remains cartilaginous ; but I 

 infer that a cartilage always extended from the anterior margin of the 

 coracoid to the anterior extremity of the scapula, and, by ossification 

 of such cartilage, the Plesiosaurian shoulder girdle would become 

 Elasmosaurian."* 



This precoracoid nature of the cartilage is contested by Koken : 

 "The view that the bone considered to be the scapula in the Plesio- 

 saurs also includes the precoracoid is supported neither by compar- 

 ison nor observation. In no reptile has there been shown to be a 

 union of the scapula with the precoracoid more intimate than its 

 union with the coracoid, and its fusion with the scapula without 

 union with the coracoid would be remarkable. I agree quite with 

 Haur, who considers the forked bone of the turtles to be the scapula 

 only, its two branches being homologous with those of the plesiosau- 

 rian scapula. Seeley believes that the precoracoid was cartilaginous, 

 connecting the front end of the coracoid with the anterior end of the 

 scapula, and that this became ossified in the Elasmosaurians in such 

 a way that it was separated from the coracoid by suture, but was 

 fused with the scapula. That is, it is the precoracoids and not the 

 scapulae which meet in the middle line. It seems to me that this 

 would be the method of extension and ossification of the scapulae. 

 Why, then, should we call in the aid of a cartilaginous precoracoid as 

 an unknown quantity, which later becomes indistinguishably fused 

 with the scapula ?"f 



The precoracoid arises from a distinct ossificatory center, when 

 ossified. Is it reasonable to suppose, then, if this is a distinct ele- 

 ment, that any such mode of extension of the scapula as Andrews has 

 shown to be the case in Cryptoclidus oxoniensis\ would occur if there 

 was really a union of scapula and precoracoid? He shows clearly 

 that the scapula increases in length peripherally, and not by the addi- 

 tion of an ossified cartilage. If it arose from a distinct center, as it 

 must, one would certainly expect to find it in a separated condition 

 in the young animal, but this was not the case in the ones that 

 Andrews examined. Furthermore, in what possible way could a carti- 



•Proc. Roy. Soc. Lond. li, p. 138, 1892. 



tKokcn. Zetttchr. Dentscben Geol. GeteJIacb. 1S93. xlv, 346. 



JAnn. Man. Nat. Hbt. 1895. 



