48 PKOCEEDLNGS OF THE GEOLOGICAL SOCIETY. 



and some examples show traces of a primitive composition, of two 

 similar halves. These are circumstances, particularly the former, 



Fig. 4. — Lacertilian Shoulder-girdle, ventral view. (After Parker.) 



xst, the xiphisternal ; the other letters as in the preceding figures, 



which appear to me to challenge its title to being a true interclavicle. 

 But if not interclavicle, is there any known skeletal element with 

 which it may be identified ? I suggest that the omosternum (W. K. 

 Parker) of existing Batrachia (figs. 5, 6) is such an element. This,, 

 often confounded with the true interclavicle (of Lizards), is shown by 

 Parker to consist primitively of two pieces, each segmented off from 

 its corresponding epicoracoid, which later coalescing form the azygos- 

 keystone of the arch. Its origin as a cartilaginous endoskeletal part 

 forbids the identification of the Batrachian omosternum with the 

 interclavicle of Lizards and Chelonia. 



In the extant Australian Batrachian Calamites cyaneus (fig. 6), 

 the omosternum is represented by Parker as permanently retaining 

 traces of its primitive duality, and sending backwards from each 

 half a small plate on the deep aspect of the praecoracoid. Its deep 

 position together with indications of an originally dual composition 

 are, I submit, good grounds for assigning a high degree of probability 

 to the homology of the median element of the shoulder-girdle in Plesio- 

 saurus with the Batrachian omosternum. So far, then, as our pre- 

 sent knowledge allows an opinion, we are warranted in considering 



