256 THEORY OF THE EARTH. 



and thick, while the other bones are flattened, 

 and closely set like the stones in a pavement, so 

 as to form, when enveloped with the skin, fins of 

 a single piece, almost incapable of bending ; ana- 

 logous, in short, both as to use and organization, 

 to those of cetacea. These reptiles have lived in 

 the sea ; on shore, they could only at most have 

 crept in the hobbling manner of seals ; at the same 

 time after they have respired elastic air. 



The remains of four species have been found : 



The most extensively distributed (/. commu- 

 nis) has blunt conical teeth ; its length sometimes 

 exceeds twenty feet. 



The second (/". platyodori), which is at least as 

 large as the former, has compressed teeth, with 

 round and bulging roots. 



The third (7. tenuirostris], has slender and 

 pointed teeth, and the muzzle thin and elon- 

 gated. 



The fourth (/. inter medius], is, as its name 

 implies, intermediate between the last species and 

 the common, with respect to the form of its teeth. 

 The two latter species do not attain half the size 

 of the two first. 



The plesiosaurus, discovered by Mr Cony- 

 beare, must have appeared still more monstrous 

 than the ichthyosaurus. It had the same limbs, 

 but somewhat more elongated and more flexible ; 

 its shoulder and pelvis were more robust ; its 



