on the Fossil Genera Icht/ij/osaurus and Plesiosaurus. 115 



whicli these occupy is completely altered^ and the lower portion of it is as it 

 were broken through ; those bones which form the inferior margin of these 

 lower fossas being wanting:, — namely^ the prolongation of the jugal (c) behind 

 the orbitj and the bone (/>) between this and the tympanal portion of the 

 temporal, or os quadratum ; and the tympanal bone itself being so placed as 

 to hang as it were loosely among the other bones of the head_, by its upper 

 portion, instead of wedging itself in among them as in the crocodile. 



The temporal fossae in the lacertae, on the other hand, answer entirely to 

 the upper fossae in the crocodile ; with the exception already stated, namely, 

 that, in order to give room to a much more powerful temporal muscle, they 

 have a much greater proportional extent. 



Having in my former memoir compared the head of the ichthyosaurus prin- 

 cipally with that of the crocodile, I at once perceived that the post-orbital 

 holes on the top of the skull of the former animal corresponded exactly, in 

 positioDj, and in the bones by which they are surrounded, with the upper fossae 

 in the crocodile : but as these appeared, in the accounts of the crocodile 

 which I had read, to be considered as peculiar to that animal, I did not carry 

 my researches further. Having since ascertained the true character of these 

 fossae, as being the principal temporal fossae, and as answering to those of the 

 other lacertae, I must of course retract any inferences deduced from having 

 considered this part as peculiarly approximate to the crocodilian type ; for it 

 will be found, in truth, that it presents a slight modification only of the usual 

 position of the temporal fossae, and indeed agrees more closely with the skulls 

 of other lacertae than with that of the crocodile ; inasmuch as the fossse are 

 larger than in the crocodile, and the contour of the surrounding bones, espe- 

 cially the forked outline of the back of the parietal bone, is similar in cha- 

 racter to the lacertian type ; to which, therefore, rather than to the crocodile, 

 the analogies derived from hence must, so far as they go, incline us to refer 

 the present genus. — But when it is considered how much wider variations exist 

 in this part between different species of crocodile (the sclerops for instance 

 and the gavial), the differences between this last species and the ichthyosaurus 

 will not, 1 think, appear of very considerable weight. In the remaining part 

 of the head we find distinctions of much more obvious and striking importance, 

 which give to its posterior portion a character strongly approximating to the 

 crocodile, and very widely removed from the other lacertae. 



I have not been able to ascertain whether the lower fossae* of the crocodile 



* The fossa, however, connected with the external opening of the tympanal cavity (see the fol. 

 lowing paragraph and Plate XVI.) j appears also to have an opening anteriorly (behind /)) j 

 and this anterior opening may perhaps represent the fossa in question. 



<J2 



