78 Professor Owen an British Fossil "Reptiles, 



the bodies of the vertebrae ; the complicated pectoral arch \ 

 the sternum, and complete abdominal cincture of ribs,* &c. 

 The circle of numerous imbricated sclerotic bones reaches its 

 maxunum of development in the ichthyosaurus ; but this is an 

 exaggeration of a structure feebly shadowed forth in some 

 existing saurians, and more strongly shewn in birds, rather 

 than a repetition of the simple bony sclerotic cup in fishes. 

 By no known forms of fossil animals can we diminish the wide 

 interval which divides the most sauroid of fishes from an ich- 

 tht/osaurus. 



This most extraordinary reptile is a singular compound, in 

 which ichthyic, cetacean, and ornithic characters are engrafted 

 upon an essentially saurian type of structure. The ichthyo- 

 saurus is, therefore, just such a form of animal as might be 

 expected, were specific forms unstable, to demonstrate a mu- 

 tation of characters, or some tendency towards a progressive 

 development into a higher and more consistent type of organ- 

 ization. Nor is the field for testing the transmutation theory 

 less ample than the subject is favourable. We have the op- 

 portunity of tracing the ichthyosauri, generation after genera- 

 tion, through the whole of the immense series of strata which 

 intervene between the new red sandstone and the tertiary de* 

 posits. Not only, however, is the generic type strictly ad- 

 hered to, but the very species, which made its first abrupt 

 appearance in the lowest of the oolitic series, maintains its 

 characters unchanged and recognizable in the highest of the 

 secondary strata. In the chalk formations, for example, the 

 genus ichthyosaurus quits the stage of existence as suddenly 

 as it entered in the lias, and with every appreciable osteologi- 

 ^ical character unchanged. 



Of the different species of the ichthyosaurus, founded upon 

 minor modifications of the skeleton, several appear contempo- 

 raneously in the strata w^here the genus is first introduced ; 

 and those which remain the longest manifest as little change 

 of specific as generic characters. There is no evidence what- 

 ever that one species has succeeded, or been the result of the 



^-' This structure proves that the mode of generation of the ichthyomurus 

 must have resembled that of the crocodile, and not that of the batrachians 

 or fishejr. 



