ANNIVERSARY ADDRESS OF THE PRESIDENT. 47 



K. Parker has shown, the true clavicles are to be found in the 

 episternal elements of the plastron (dermal ossifications) ; the reputed 



Fig. 3. — Shoulder-girdle of Chelone mydas, dorsal view, (Slightly 

 altered, after Parker.) 



cl, the clavicle; ic, the inter clavicle ; per, the prascoracoid ; c, the coracoid 



clavicles, which are parts of the cartilaginous endoskeleton, are then 

 the praccoracoids ; whence it follows that there is a degree of 

 probability, almost amounting to proof, that the corresponding bar in 

 Plesiosaunis is also praecoracoid, and thus evidence of the presence of 

 true clavicles in this genus and, I may add, Nothosaurus, fails ; so 

 that in the present state of our knowledge on this point we are 

 warranted in considering clavicles to be wanting in Sauropterygia. 



There remains for comparison one more piece, the azygos median 

 bone called episternum or interclavicle. 



In Ichthyosaurus (fig. 1) this T-shaped bone so closely agrees in its 

 form and its relations with the interclavicle of existing Lizards and 

 Chelonia that its homology with this is unimpeachable. Can the same 

 be maintained of the analogous piece in the shoulder-girdle of Plesio- 

 saurus ? I submit that its homology with the lacertilian interclavicle 

 is more than doubtful. The Lizard's interclavicle is a membrane 

 bone ; it lies superficially (fig. 4). In Ichthyosaurus, as in extant 

 Lizards, it lies at the under surface of the other (endoskeletal) elements 

 of the girdle. In Plesiosaunis (fig. 2) the reputed clavicle lies above 

 the praecoracoids and coracoids, upon their upper or visceral surface, 



