80 Professor Owen on British Fossil Reptiles. 



lance the elongation of the jaws, the neck at the same tune 

 shrunk to nearly its former Ichthyosaurian proportions, with 

 some slight modifications of the Plesiosaurian type of the ver- 

 tebrae ; if a further development and a more complete sepa- 

 ration of the digits of the fore and hind members were to take 

 place, so that they miglit serve for creeping as well as swim- 

 ming ; if the exposure of the surface to two different media, 

 and of the entire animal to perils of land as well as of sea, 

 were to be followed by the ossification of certain parts of the 

 skin, and the acquisition, by this change, of a dermal armour, 

 such we might conceive to be the leading steps in the trans- 

 mutation of the Plesiosaur into the Teleosaur. 



And if the three forms of extinct Saurians, whose changes of 

 specific and generic have thus been speculated on, had actually 

 succeeded each other in strata successively superimposed in the 

 order in which they havehere been hypothetically derived from 

 one another, some colour of probability might attach itself to 

 this hypothesis, and there would be ground for searching more 

 closely into the anatomical and physiological possibilities of 

 such transmutations. Ichthyosaurus, Plesiosaurus, and Teleo- 

 saurus are, however, genera which appeared contemporaneous- 

 ly on the stage of vital existence ; one neither preceded nor 

 came after the other. How the transmutation theory is to 

 be reconciled to these facts is not obvious ; nor to these others, 

 viz. that the Teleosaur ceases with the oolite, while the Icli- 

 thyosaur and Plesiosaur continue to coexist to the deposition 

 of the chalk, and disappear together alike unchanged ; the 

 Ichthyosaur manifesting as little tendency to develope itself 

 into a Plesiosaur, as this to degrade itself into the more fish- 

 like modification of the Enaliosaurian type. 



If it were urged that the Strep tospondi/lus, or crocodile with 

 ball and socket vertebrae, of which the remains occur in later 

 secondary strata, when the Teleosaur had ceased to exist, 

 might be a modification of the apparently extinct Amphicae- 

 lian crocodile, in which the vertebrae had undergone a pro- 

 gressive development, analogous to that by which the bicon- 

 cave joints of the vertebrae of the tadpole are actually con- 

 verted into the ball and socket joints of those of the mature 

 frog, the facts of both geology and anatomy again oppose 



