PART II. CHAPTER XVIII. 227 



Reptiles of the Lias. 



cion.and Chimsera (see a. Fig. 226.). In both of these genera, 

 the posterior concave face is armed with small spines like that 

 of the fossil Hybodus (Fig. 225.), one of the shark family found 

 fossil at Lyme Regis. Such spines are simply imbedded in 

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

 Dr. Buckland, " as in the Chimcera (Fig. 226.), to raise and 

 depress the fin, their action resembling that of a moveable mast, 

 raising and lowering backwards the sail of a barge."* 



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. The 

 genus Ichthyosaurus, or fish-lizard, is not confined to this form- 

 ation, but has been found in strata as high as the chalk-marl and 

 gault of England, and as low as the muschelkalk, a formation 

 which immediately succeeds the lias in the descending order.f 

 It is evident from their fish-like vertebrae, their paddles, resem- 

 bling 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 carni- 

 vorous ; and the half-digested remains of fishes and reptiles, 

 found within their skeletons, indicate the precise nature of their 

 food4 Mr. Conybeare was enabled, in 1824, after examining 

 many skeletons nearly perfect, to give an ideal restoration of 

 the osteology of this genus, and of that of the Plesiosaurus. 

 The latter animal had an extremely long neck and small head, 

 with teeth like those of the crocodile, and paddles analogous to 

 those of the Ichthyosaurus, but larger. It is supposed to have 

 lived in shallow seas and estuaries, and to have breathed air like 

 the Ichthyosaur, and our modern cetacea.|| Some of the rep- 

 tiles above mentioned were of formidable dimensions. One 

 specimen of Ichthyosaurus playtyodon, from the lias at Lyme, 

 now in the British Museum, must have belonged to an animal 

 more than twenty-four feet in length, and another of the Plesio- 

 saurus, in the same collection, is eleven feet long. The form of 

 the Ichthyosaurus may have fitted it to cut through the waves 

 like the porpoise ; but it is supposed that the Plesiosaurus, at 

 least the long-necked species (Fig. 228.), was better suited to 

 fish in shallow creeks and bays, defended from heavy breakers. 



* Bridgewater Treatise, p. 290. t Buckland, Bridgew. Treat., p. 168. 



t Ibid. p. 187. Geol. Trans., Second Series, vol. i. pi. 49. 



II Conybeare and De la Beche, Geol. Trans.; and Buckland, Bridgew. 

 Treat., p. 203. 



