252 Mr. J. W. Hulke. On the Shoulder Girdle, 



precoracoid, actually scapula, as Professor H. G-. Seeley deems it to- 

 be, it must be either acromion, or prescapula. The former was 

 Conybeare's idea ; but Cuvier pointed out this involved an extension 

 of a scapular process mesially inwards towards its fellow of the other 

 side, a construction not known in any Vertebrate ; and the same 

 objection lies against a prescapular interpretation. 



An objection that might be advanced against homologising the dorsal 

 process in the Plesiosaurian shoulder girdle with the Testudinate 

 shoulder blade, is that it does not ascend so immediately from above 

 the glenoid fossa as does the latter. This is true, but I offer the 

 suggestion (for what it may be worth) that the clue to this difference 

 lies in the dwarfing of this process in Plesiosaurus ; whilst the growth 

 of the precoracoid has been relatively rapid, with the effect of carrying 

 forward with it the free process of the scapula, which gives to that 

 part of the " anterior bone " somewhat the semblance of a third process. 

 In connection with this it should not be overlooked that in Testudi- 

 nata before ossification of the shoulder girdle is complete that 

 part of the glenoid mass of cartilage which belongs to the scapula 

 and precoracoid stands off, process-like, from the angle which 

 marks the junction of the two components of the " anterior bone," 

 thus presenting a slight resemblance to that which obtains perma- 

 nently in Plesiosauria. 



There remains for discussion the homology of the bony piece or 

 pieces which I have suggested may be omosternal, but which 

 Professor H. G. Seeley contends are clavicles and interclavicle. 

 They lie upon the upper or visceral aspect of the precoracoids 

 (Professor Seeley), therefore within the outer bony frame of the 

 chest; and only to an extent varying in different genera in a 

 ventral view are they visible between the anterior mesial ends of 

 hese bones (precoracoids mihi), in advance of the coracoids, above 

 and behind the antero-internal angles of which they are produced 

 backwards to an extent hidden in most skeletons. 



In Plesiosaurus (type) they form a plate which is more exposed, 

 in a ventral view, than in Pliosaurus (B. Owen) in which only a 

 very small part of the omosternum is apparent in the vacuity between 

 the antero-internal angles of the coracoids ; whilst in Colymbosaurus 

 (H. G-, Seeley), the omosternum, if it exist, is completely hidden by the 

 precoracoids (Professor Seeley). Fig. 12, p. 447, ' Geol. Soc. Quart. 

 Journ.,' 1874. 



It was this undisputedly deep position of the elements in question, 

 unknown as regards interclavicle and clavicles in any other Verte- 

 brata, that weighed with me against accepting the interclavicular 

 hypothesis held by several previous writers : for I then knew, and I 

 still know, no Vertebrate skeleton in which an interclavicle is thus 

 deeply situated under cover of the other bones of the pectoral frame. 



