104 Prof. H. G. Seeley. The Shoulder Girdle and [June 15, 



Ohelonian bone, recent or fossil, which I have examined, so far as can 

 be judged from the ossified bone, and it is closely comparable to the 

 conditions of ossification of Plesiosaurs on the one hand, and of 

 Amphibians on the other, in which the original cartilage of the 

 humerus and femur becomes sheathed by an external layer of bone, 

 leaving the cartilage in the position of an epiphysis, which penetrates 

 more or less conically from the articular joint of the bone into the 

 sheath, becoming an ossification which sometimes blends with it, and 

 is sometimes for a time separate in the young state. 



There is no evidence advanced that two distinct cartilages enter 

 into the composition of the part of the glenoid fossa formed by this 

 bone, and I affirm that its two rays are sheathed separately, because, 

 owing to the original form of the cartilage, it would not be conceiv- 

 able that the sheaths should be formed in any other way. But in his 

 account of the young Spliargis, Rathke expressly states that the two 

 sheaths -coalesce when they meet each other, and therefore form one 

 sheath, without any indication of a primitive separation between the 

 two rays of ossification which afterwards became obliterated. Just 

 as the ossification of the humerus of a Plesiosaur in three portions 

 does not make that bone any the less a humerus, so the ossification of 

 the scapula in a Chelonian in two or three parts, of which the articular 

 part was originally cartilaginous and two are the coalesced ectosteal 

 sheaths, does not seem to me to make that bone any the less a scapula 

 in Chelonians. There is therefore no proof that the bone in Che- 

 lonians is anything but a scapula, or that the precoracoid is present 

 in the Chelonian skeleton, any more than in the Plesiosaurian 

 skeleton. 



It only remains to point out that the ascending process of the Plesio- 

 saurian scapula is homologous with that which is developed backward 

 in the Nothosaurian scapula (figs. 6 and 7), and I have no doubt that 

 both are homologous with the blade of the scapula in Ichthyosaurs 

 and Anomodonts. But, in proportion as this scapular blade is de- 

 veloped, so does the scapula acquire a position which is posterior to the 

 coracoid ; while that position is the result of a special development of 

 a new structure which ascends in these animals external to the ribs, 

 above the humeral articulation. Its development is accompanied in 

 Anomodonts by an atrophy of all that part of the scapula which had 

 originally extended in advance of the articulation. This view may 

 be hereafter established by further evidence. If the older view, which 

 Mr. Hulke puts forward, should be preferred, there would still remain 

 the evidence that the precoracoid has no existence in Chelonia, and 

 therefore inferentially has no existence in Sauropterygia. So that 

 the bone which extends in Plesiosaurs anterior to the coracoid would 

 still be the scapula, and the shading which Mr. Hulke places upon it, 

 dividing it into precoracoid and scapula, would be a delusive indica- 



