46 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



with the scapula. This interpretation seems to be supported by an 

 analogous bar in the amphibian shoulder-girdle. This, considered 



Fig. 2. — Shoulder-girdle of Plesiosaurus (type), ventral view. 



osty the omosternum ; pc, the prsecoracoid ; sc, the scapula ; co, the coracoid ; 

 H, the humerus. 



praecoracoid by W. K. Parker, regarded as clavicle by Wiedersheimer, 

 thought to comprise both praecoracoid and clavicle by Hoffmann, is 

 primitively a simple cartilaginous rod extending ventrally from the 

 scapula towards the middle line of the body. Later, an ectosteal splint 

 forms on its front border, and, growing, more or less sheathes it, the rod 

 itself meanwhile undergoing a variable amount of endosteal ossifi- 

 cation ; and when these two processes of ossification are completed, a 

 slight longitudinal furrow may be the only trace of the dual origin 

 of the bar. It is evident that the ectosteal splint alone can have any 

 pretention to homology with the clavicles of higher vertebrates ; and 

 I imagine that its intimate association with the cartilaginous rod 

 (which is segmented off by longitudinal fissure from the coracoid and 

 is therefore unmistakably praecoracoid) was one of the considerations 

 that led Prof. Parker to regard the bar in its entirety as praecora- 

 coid, and to deny the occurrence of true clavicles in Batrachia, a view 

 which has much in favour of it. In Chelonia (fig. 3), however, the 

 ambiguity, so puzzling in Batrachia, disappears. They also have a 

 similarly placed ventral bar, which, except in very immature indi- 

 viduals, is (as in Plesiosaurus) confluent with the glenoid part of the 

 scapula, from which it extends inwards towards the middle line of 

 the body. Early comparative anatomists regarded this as an acro- 

 mion or clavicle, the view, I think, still held by Prof. R. Owen. Of 

 its homology with the analogous bar in Plesiosaurus and Nothosaurus 

 I myself have no doubt ; and if it be clavicle in the Chelonian it is so 

 in Plesiosaurus and Nothosaurus. and thus clavicles would be present 

 in Sauropterygia as in Ichthyopterygia. But in Chelonia, as Prof. W. 



