THEORY OF THE EARTH. 255 



only vegetable impressions of large arundinaceaa, 

 bamboos, palms, and other monocotyledonous 

 plants, we come to the different strata of the de- 

 posit which has been named the Jura limestone, 

 on account of its forming the principal nucleus of 

 that chain of mountains. 



It is here that the class of Reptiles assumes its 

 full development, and shews itself under the most 

 varied forms and gigantic sizes. 



The middle part, which is composed of oolites 

 and lias, or of grey sandstone containing gryphites, 

 contains the remains of two genera, the most ex- 

 traordinary of all, which have combined the cha- 

 racters of the class of oviparous quadrupeds with 

 organs of motion similar to those of the cetacea. 



The ichthyosaurus *, discovered by Sir Everard 

 Home, has the head of a lizard, but prolonged 

 into an attenuated muzzle, armed with conical 

 and pointed teeth ; enormous eyes, the sclerotica 

 of which is strengthened by a frame consisting of 

 bony pieces ; a spine composed of flat vertebrae, of 

 a depressed circular form, and concave on both 

 surfaces like those of fishes ; slender ribs ; a ster- 

 num and clavicles like those of lizards and orni- 

 thorynchi; a small and weak pelvis; and four 

 limbs, of which the humeri and femurs are short 



See my " Recherches," vol. v. part ii. p. 447. 





