xiv MAEI^'E EEPTILES OF THE OXFOED CLIT. 



clearly a member of the longipinnate group, the fore paddle attains great width 

 through the addition of at least two rows of supplementary ossicles on the radial side 

 and one on the ulnar side. 



OpMMlmosaurus, with its powerful tail-fin, pointed head, and porpoise-like body, 

 must have been a very sAvift and powerful swimmer, even for an Ichthyosaur, and 

 probably lived in the open sea like most of the Toothed Whales of to-day ; like them, 

 too, it Avas no doubt capable of diving and swimming at considerable depths, the 

 structure of the auditory apparatus, in the opinion of UoUo *, being specially adapted 

 for use under great pressures such as the animal would be subjected to at some 

 distance beneath the surface. Although in its mode of life 0])hthalmosaurus probably 

 did not differ greatly from other members of the order, the reduction of the dentition 

 indicates that its food probably differed from theirs, though of its nature nothing is 

 known. 



All the Plesiosaurs described in the present volume are members of the Family 



Elasmosauridfe, characterised especially by the structure of the shoulder-girdle, in 



which, in the adult, the scapulae meet in a median symphysis, which is continuous 



posteriorly with the symphysis of the coracoids. The ingrowth of the scapulae 



towards the middle line takes place beneath the clavicular arch, which thus comes to 



lie on the visceral surface of the ventral rami of the scapulee, which usurp its functions. 



The consequence of this is, that the clavicles and interclavicles undergo reduction in 



varying ways. In some genera all the elements of the clavicular arch persist in a 



reduced form, in others tlie clavicles or interclavicle may dwindle away to mere 



vestiges. These varied conditions of the clavicular arch supply some of the chief 



characters employed in defining the different genera. If Professcr Seeley'sf restoration 



of the shoulder-girdle of Eretmosaurus rvgosus be correct, it would appear that the 



arrangement of the coracoids and scapulfe found in the Elasmosauridae had already 



come into existence in the period of the Lower Lias ; and, at any rate, it seems 



certain that it had done so in the Upper Lias, for Mr. D. M. S. Watson % has lately 



described from beds of that age at Whitby, a shoulder-girdle of Pleswsanrtis Jwmalo- 



sjjondylus (referred by him to a new genus Microcleidus), in which the form and 



* L. Dollo, " L'aiidition ehez les Ichtliyosauriens,'' Bull. Soc. Beige Geol. etc. vol. ssi. (1907) p. 157. 



t Quart. Jovirn. Geol. Soc. vol. sxx. (1874) p. 445. 



t Mem. & Proc. Manchester Lit. & Phil. Soc. vol. liv. (1909-10) no. 4, p. 4. 



