Professor Owen on British Fossil Beptiles. 79 



transmutation of a former species. The tenuirostral ichthy^ 

 osaurus existed at the same time, and under the same exter- 

 nal influences, as the stronger and shorter jawed Ichthyosau- 

 rus communis ; just as the tenuirostral Dclphinus Gangeticus 

 co-exists at present with the short-jawed porpoise. 



If the relative periods of existence of the different Enalio- 

 saurian reptiles were not well ascertained, and room were 

 allowed for conjecture as to their successive appearance on 

 this planet, it would be as easy as seductive to speculate on 

 the metamorphoses by which their organic frame-work, influ- 

 enced by varying conditions, during a lapse of ages, might 

 have been gradually modified, so as to have successively de- 

 veloped itself from an ichthyosaur to a plesiosaur, and thence 

 to a crocodile. 



We may readily conceive, for example, the fish-like charac- 

 ters of the vertebral column of the ichthyosaurus to have been 

 obliterated by a filling up of the intervertebral cavities 

 through ossification of the intermediate elastic tissue, and the 

 plesiosaurian type of vertebra to be thus acquired. The nor- 

 mal digits of the fin might be supposed to become strengthen- 

 ed and elongated by more frequent reptation on dry land, 

 and thus to cause an atrophy of the supernumerary fingers ; 

 phalanges of a more saurian figure might have been produced 

 by the confluence of a certain number of digital ossicles ; the 

 head might be shortened by a stunted growth of the intermax- 

 illary bones, and thus be reduced to plesiosaurian proportions. 

 The teeth might become more firmly fixed by the shooting of 

 bony walls across their interspaces, as in the young crocodiles. 

 If we now elongate the bodies of the vertebrae, reduce some 

 twenty pairs of anterior ribs to hatchet bones, place the fore- 

 paddles at a corresponding distance from the head, and the 

 hind-paddles proportionally nearer the end of the tail, little 

 more will be required to complete the transmutation of the 

 ichthyosaur into the plesiosaur. 



If next, in adaptation to a gradual change of surrounding 

 circumstances, the jaws of the Plesiosaur became lengthened 

 to the proportions of those of the tenuirostral Ichthyosaur, but 

 at the expense of maxillary, instead of the intermaxillary bones, 

 preserving the socketed implantation of the teeth ; if, to ba- 



