248 Mr. J. W. flulke. On the Shoulder Girdle 



sidered precoracoid by C. Gegenbaur, Fiirbringer, Huxley, W. K. 

 Parker, and others, including myself; and is named clavicle by 

 Gotte, Hoffmann, and others, their followers. 



The chief argument used in my address, from which Professor H. 

 G. Seeley quotes, that this piece in the Plesiosaurian shoulder girdle 

 is really a precoracoid, is the very close agreement noticeable between 

 the Plesiosaurian and Testudinate girdles, a correspondence not merely 

 general, but also of their respective parts ; so that, if the anterior 

 ventral ray in Testudinata is a precoracoid, this affords a very strong 

 presumption that the corresponding ray in Plesiosauria is also pre- 

 coracoid. This presumption appears to me to amount as nearly to 

 proof as the nature of the comparison of these parts admits ; but since 

 Professor H. G. Seeley considers " the evidence insufficient to sustain 

 the interpretation," and since, on re-examination, my former conclu- 

 sion is confirmed, I shall amplify and re-state it. 



In comparing skeletons of fossil with those of extant animals, 

 absolutely complete proof of the essential identity of their several 

 parts is, in particular instances, unattainable, and we have to accept 

 for it presumption so strong as not to allow reasonable doubt. As 

 regards living animals, we have the great aid of embryology to 

 illumine the facts of the mature skeleton ; but this valuable aid is, if 

 we except a few very rare instances (e.</., larval, concurrent with 

 adult forms, as in the Batrachial fauna of the Braun-Kohlen formation 

 of Rhenish Prussia), wanting in regard to fossil animal remains, 

 which are so often fragmentary and otherwise imperfect and incom- 

 plete. 



In Testudinata it is a matter of common elementary knowledge 

 that the primary shoulder girdle (primitively a three-rayed cartilage, 

 having at its centre a hollow for the reception of the caput humeri, 

 the fossa glenoidalis) appears, when ossification is completed, to com- 

 prise two bones only. Of these, one, the posterior ventral ray, 

 contributes the posterior part of the fossa glenoidalis, and sends an 

 expansion inwards towards the mesial line of the body. It is by 

 common consent coracoid, and it requires no further notice. 



The other bone has an angulated form, of which one branch ascends 

 to be attached ligameutously at the under surface of the carapace ; 

 whilst the other branch bends ventrally inwards, in front of the 

 posterior ray, approaching closely that of the other side, and is 

 mostly attached by its mesial end ligamentously to the entoplastron. 



It is respecting the homology of this (anterior) ventral branch that 

 there remains any difference of opinion, since all agree that the 

 dorsally-directed branch is scapula. 



As already mentioned, these two branches appear to constitute one 

 bone, which meets the coracoid in the glenoid mass, and with the 

 coracoid forms the glenoid fossa. Of this hollow the apparently 



