36 



WATER REPTILES OF THE PAST AND PRESENT 



constant in reptiles, a more or less T-shaped bone underlying the 

 coracoids where they join, or the breast bone; but there were some 

 reptiles that lost it, the dinosaurs and pterodactyls, for instance. 

 In the turtles both the clavicles and the interclavicle form a part 

 of the under shell or plastron. 



Fig. iq. — Scapula {sc), coracoid {cor), and metacoracoid {mcor) of Dimetrodon 



The cleithrum is known in only a few of the old reptiles; it 

 is a more or less slender bone which lies along the upper front 

 margin of the scapula, articulating at its lower end with the upper 

 end of the clavicle on each side. 



