BRITISH FOSSIL REPTILES. 45 



ties of the species ; thirdly, with relation to the localities and 

 extent of strata through which the several species are distri- 

 buted, or to which they may be restricted. 



Part 1. Enaliosauria. — General Characters of the Order. 



The Enaliusaurs were vertebrate, air-breathing and cold- 

 blooded animals; referrible therefore to the great class of Rep- 

 tilia in the Cuvierian system ; and indicative of a primary 

 modification of the typical structure of that class, by which 

 they were fitted more especially for a marine life. 



The proof that the Enaliosaiirs respired atmospheric air im- 

 mediately is afforded by the position and structure of the nasal 

 passages, and by the osseous mechanism of the thoracic- abdo- 

 minal cavity. 



The evidence that they were cold-blooded animals, reposes 

 on the unanchylosed condition of the elementary osseous pieces 

 of the occiput and other cranial bones, of the lower jaw, and 

 of the vertebral column : the laws of organic coexistence 

 justify the conclusions from these conditions of the osseous 

 system that the heart was adapted to transmit only a part of 

 the circulating blood through the respiratory organs. 



The peculiar modifications of the saurian type-, or the special 

 Enaliosaurian characteristics, consist in the absence of the ball 

 and socket articulations of the bodies of the vertebrae ; the 

 position of the nostrils at or near the summit of the head ; 

 their separated hsemapophyses ; the numerous, short, and flat 

 digital bones, which must have been enveloped in a simple, un- 

 divided, tegumentary sheath, forming in both the fore and hind 

 extremities a fin resembling in external appearance the paddles 

 of the Cetacea. 



Other genera of Fossil Saiiria were aquatic and marine, and 

 consequently possessed extremities modified for swimming, as 

 are indeed those of the marine Chelonia of the present day, and 

 in a less striking degree the feet of the Crocodiles among exist- 

 ing Sauria. But those reptiles only ought to be regarded as true 

 Enaliosaurs which combine limbs fitted for swimming with the 

 cranial and vertebral characters above defined*. 



The Enaliosauria offer two principal modifications of their 

 anatomical structure, of which the genera Plesiosaiirus and 



* The saurian system of M. H. v. Meyer, which includes the Teleosaurs and 

 Mososam-s, with the true Enaliosaurs, on account of the modifications of the 

 locomotive extremities, is not attended with any advantages compensatory of 

 its extremely artificial nature. The bones of the extremities are less available 

 than the vertebra? or teeth in indicating the generic and specific characters, or 

 the true affinities of the individual Saurian to which they may have belonged. 



