THE ICHTHYOSAURI!^. 423 



THE PLESIOSATJRUS, (from TrXeo-toj/, akin to, and a-avpa a 

 lizard.) This reptile, as its name imports, was more nearly 

 allied to the lizard than the fish, especially as regards the 

 character of its vertebrae. Like the Ichthyosaurus, it united 

 in its structure various types of organisation ; it had the head 

 of a lizard, the teeth of a crocodile, a neck of enormous 

 length, resembling the body of a serpent, a back and tail 

 having the proportions of an ordinary quadruped, the ribs of 

 a chameleon, and the paddles of a whale. It is supposed to 

 have so far differed in its habits from the Ichthyosaurus, that 

 while that creature sought the depths of the ocean, the 

 Plesiosaurus is considered to have floated upon or near the 

 surface of shallow waters ; arching back its long neck like 

 a swan, and occasionally darting it at the fish which swam 

 within its reach. Ten species of this genus are found in the 

 liasic beds, but several others are special to the higher stages 

 of the oolitic group. 



One of the most remarkable species is the Plesiosaurus 

 rugosus, which was recently presented to the British Museum 

 by the Duke of Eutland, and described by Professor Owen : 

 it is characterised by certain rugose markings on the 

 vertebrae and other parts of the skeleton. Figure (fig. 291) 

 is a faithful representation of the specimen ; , is two 

 vertebrae, with rugose edges. The most perfect specimen of 

 plesiosaurus yet brought to light is one recently discovered 

 at Whitby, which has been purchased at the price of 230Z. 

 for the Fitzwilliam Museum, at Cambridge. This saurian 

 has been described by Professor Owen under the name 

 P. grandipinnis, from the large size of its paddles. (See 

 fig. 203.) 



Erom the state of perfection in which these and other 

 specimens have been discovered, many of the skeletons con- 

 taining between their ribs the scales and other remains of 

 fishes, it is inferred that they must have been suddenly 

 destroyed, and entombed in the sediment in which they are 

 now enveloped. 



ORIGIN or THE OOLITE AND LIAS. It is supposed that 

 the ocean which deposited these groups was liable to sudden 

 changes ; the varied nature of the beds probably justifies this 

 inference, as argillaceous, arenaceous, and calcareous strata 

 are found to succeed each other, the clay usually forming 



