in Ickthyosauria and Saiiropterygia. 251 



reasonably be doubted each is precoracoid (clavicula, Gotte). In 

 Testudinata all accept the dorsal ray as representing the blade of the 

 scapula in other Reptilia ; but in Plesiosauria the analogous dorsal 

 ray, Professor H. G. Seeley contends, is not an homologous structure. 

 Were it so, then, obviously, [the agreement of the two girdles in regard 

 to this element would be seriously compromised. 



The ground on which Professor H. G. Seeley rejects the idea that 

 the dorsal process in the Plesiosaurian girdle is homologous with the 

 dorsal ray'in the Testudinate girdle, is stated by him in the following 

 passage : " In Chelonians the ascending process of the scapula 

 extends dorsally towards the vertebras, while in Sauropterygia it 

 extends backwards above the glenoid articulation for the humerus, 

 and there is no evidence that these structures are homologous" (c/. 

 ' Koy. Soc. Proc.,' vol. 51, 1892, p. 122). 



It is manifest from this that the backward slant of the Plesiosaurian 

 dorsal scapular process is Prof essor H. G. Seeley 's chief (and only stated) 

 reason for rejecting its homology with the Chelonian shoulder blade. 

 Can this reason be accepted as a sufficient warrant for such rejection ? 

 In Birds, does not the long, sword-like shoulder blade slant yet more, 

 bending back wards above and behind the glenoid fossa, in a direction 

 roughly parallel with that of the vertebral column ? In his restora- 

 tion of the Anomodont Keirognathus cordylus, has not Professor H. G. 

 Seeley given the shoulder blade a forward inclination so great as to 

 carry its free end so far in advance of the glenoid fossa as to bring 

 it against the centrum of the second vertebra, while a plane laid 

 through the glenoid fossa passes between the seventh and eighth 

 vertebrae ? Yet, notwithstanding this excessive forward slant, Pro- 

 fessor H. G. Seeley names it scapula. Between these extremes, every 

 degree of slope is observable, so that if the main fact of general dorsal 

 direction above the glenoid fossa be present, this seems to me enough 

 to justify its identification as shoulder blade, and therefore to warrant 

 it and the Chelonian shoulder blade being considered homologous 

 structures. 



Should any one still find difficulty in accepting the dorsal 

 Plesiosaurian process as shoulder blade, by reason of the somewhat 

 singular position whence it ascends,. as homologous with the ascending 

 part of the Testudinate scapula, he will find an intermediate step in 

 the Nothosaurian scapula where the corresponding dorsal process* 

 ascends directly from above the glenoid fossa. 



In other No thesauri das, e.g., in Lariosaurus, Cuvier, Neusticosaurus, 

 Seeley, to which reference has been already made, the position of the 

 root of the dorsal scapular process corresponds to that in N. mirabilis. 



Were the part in the Plesiosaurian shoulder girdle, which I call 



* The identity of which with that in Plesiosaurus has not, so far as I am aware, 

 been doubted. 



