IN THE ORDER SAUROPTERYGIA. 137 



52', but contribute, as in Plesiosaurus, the mesial border of those 

 vacuities, and articulate, underlapping it, with the hinder end of 

 the episternum, .59. The proportional characters of this element are 

 given in figs. 1 & 2. 



In thus determining the homologies of the constituents of the 

 complex bony buckler in Sauropterygia, I have exhausted every 

 subject of comparison at my command, derivable from fossil remains 

 of the group and from other Reptilian forms both fossil and recent, 

 and in the latter have had recourse to modes and stages of develop- 

 ment of the constituents of the answerable part of the frame. 



The degree in which the abdominal surface is defended by bone 

 in Sauropterygia resembles that in Chelonia. But the homology of 

 the defensive parts can be safely predicated of but a small pro- 

 portion only of the elements of the plastron. The episternum 

 (figs. 1 & 2, 59) may answer to three of the Chelonian elements, viz. 

 to the pair of bones so named, and marked es in fig. 3, and 

 to the mesial piece, s, continued backward in a pointed form, and 

 called " entosternum." But such constituents have coalesced into 

 one bone in Sauropterygia, and I have no evidence, as in Chelonia *, 

 of its development from several centres. 



One might be tempted by the size and shape of the parial elements 

 of the plastron, hs (hyosternals), in the immature tortoise (fig. 3), 



Fig. 3. — Development of Plastron, young Tortoise. 



to regard the broad coracoids, 52, in tigs. 1 & Z, as homologues, espe- 

 cially in the Chelonian half-developed state, when the fore and 

 outer angle is produced in direction and degree like the scapular 

 process, 51*, in Pliosaurus. But the process in the plastron of the 

 Tortoise expands as it grows, and ultimately articulates with the 

 dermo-marginal pieces of the carapace ; it is, like them, a " dermal 



* " On the Development and Homologies of the Carapace and Plastron of 

 the Chelonia " (Philosophical Transactions, 1849). 



