1893.; 



93. J Clavicular Arch in Ichthyosauria and Sauropterygia. 1(53 



in Chelonians. There is in this interpretation an element of sim- 

 plicity, because the scapula is simply inclined forward in a way to 

 which there may be some slight approximation in Chelonians, which 

 brings the two types of shoulder girdle into easy comparison. The 

 coracoids in Plesiosaurs meet each other in the median line, and this 

 condition has, I suppose, led to the atrophy and non-development of 

 such a ray as Mr. Hulke terms the precoracoid, but which I believe 

 to be a portion of the scapula,* on the hypothesis that such a ray- 

 may have once existed. 



The choice between these methods of interpretation may depend 

 upon the name adopted for the bone which most anatomists have 

 termed scapulo-precoracoid in Chelonians, for if there is a pre- 

 coracoid in the Chelonian, which is blended with the scapula, so as to 

 be inseparable from it, then there would be some ground for Mr. 

 ETulke's contention that the precoracoid was represented in the scapula 

 of Plesiosaurs, even though there would be a difference of opinion still 

 as to the position in which it was to be sought for, which would 

 depend upon views as to the way in which the Plesiosaurian girdle 

 was formed. Professor "W. K. Parker termed the inner ray of the 

 Chelonian bone precoracoid, and appears to base his interpretation 

 chiefly upon the condition of the bones in the African Ostrich. It is 

 stated (' Ray Soc.,' " Shoulder Girdle," p. 141) by the late Professor 

 W. K. Parker, of Chelone Mydas, " There is nothing that can be 

 called ' prse- ' or ' meso-scapula,' save the swollen part in front, which 

 passes uninterruptedly into the precoracoid ; this front fork forms, with 

 the scapula, a gentle arc (PL XII, fig. 3) ; it is of the same thickness, 

 nearly of the same length, and has no separate osseous centre, the two 

 bars being hardened by one ectosteal sheath." I gather that Professor 

 Parker's observations were all made upon the ripe embryo or newly- 

 hatched young. A somewhat younger specimen in the Museum of the 

 Royal College of Surgeons has by the kindness of Professor Stewart 

 been examined for me by Mr. R. H. Burne, but without showing any 

 indication of composite structure. Mr. Hulke quotes H. Rathke's 

 account of the earlier ossification of this bone, in which it was found 

 that each of the two limbs of cartilage has a distinct sheafch, and that 

 they had not quite reached each other at the stage described, in 

 some types ; while in another type the two limbs were united by 

 ossification on their inner side. I can find no evidence in this con- 

 dition that the bone which is termed precoracoid is distinct from the 

 bone which is termed scapula, nor can I find in Bathke's account of 

 the Chelonian shoulder girdle any evidence that he supposed that 

 the two rays were distinct from each other, and that each con- 

 tributed to the formation of the hollow of the shoulder joint. What 

 Rathke describes is in complete harmony with the structure of every 

 * This view has also been adopted by Professor Greorge Baur. 



M 2 



