322 REPTILES OF THE LIAS. [Cn. XXL 



appear to have been bony spines whicb formed the anterior part of the 

 dorsal fin, hke that of the Kving genera Cestracion and Chimcera (see a, 

 fis^. 414). In both of these genera, the posterior concave face is armed 



Fig. 414. 



CMnicera monsirosa* 

 a. Spine forming anterior part of the dorsal fin. 



with small spines, as in that of the fossil Hyhodus (fig. 413), one of the 

 shark family found fossil at Lyme Regis. Such spines are simply imbed- 

 ded in the flesh, and attached to strong muscles. " They serve," says 

 Dr. Buckland, "as in the Chimmra (fig. 414), to raise and depress the 

 fin, their action resembling that of a movable mast, raising and lowering 

 backwards the sail of a baige."f 



Reptiles of the Lias. — It is not, however, the fossil fish which form 

 the most striking feature in the organic remains of the Lias ; but the 

 reptiles, which are extraordinary for their number, size, and structure. 

 Among the most singular of these are several species of Ichthyosaurus and 

 Plesiosaurus (figs. 415, 416). The genus Ichthyosaurus, or fish-lizard, 

 is not confined to this formation, but has been found in sti'ata as high 

 as the lower chalk of England, and as low as the trias of Germany, 

 a formation which immediately succeeds the lias in the descending 

 order.J It is evident from their fish-like vertebrae, their paddles, re- 

 sembling those of a porpoise or whale, the length of their tail, and other 

 parts of their structure, that the habits of the Ichthyosaurs were aquatic. 

 Their jaws and teeth show that they were carnivorous ; and the half- 

 digested remains of fishes and reptiles, found within their skeletons, in- 

 dicate the precise nature of their food.§ 



A specimen of the hinder fin or paddle of Ichthyosaurus communis 

 was discovered in 1840 at Barrow-on-Soar, by Sir P. Egerton, which 

 distinctly exhibits on its posterior margin the remains of cartilaginous 

 rays that bifurcate as they approach the edge, like those in the fin of a 

 fish (see «, fig. 417). It had previously been supposed, says Prof Owen, 

 that the locomotive organs of the Ichthyosaurus were enveloped, while 

 living, in a smooth integument, like that of the turtle and porpoise, 

 which has no other support than is afforded by the bones and ligaments 

 within ; but it now appears that the fin was much larger, expanding fai 



* Agassiz, Poissons Fossiles, vol. iii. tab. C, fig. 1. 



f Bridgewater Treatise, p. 290. % ^bid. p. 168. § Ibid. p. 181. 



