206 PALEONTOLOGY 



by the parietal internally, by the mastoid and post-frontal 

 externally ; they are of an oval form, with the great end 

 forward. In their relative size and backward position they 

 are more crocodilian than lacertian. 



In the Ichthyosaurus communis there are seventeen sclerotic 

 plates forming the fore part of the eyeball. In a well-preserved 

 example, the pupillary or corneal vacuity, as bounded by those 

 plates, is of a full oval form, li" inch in long diameter, the 

 length of the plates (or breadth of the frame) being from 8 to 

 10 lines. In the same skull the long diameter of the orbit is 

 4 inches. The deep position of the sclerotic circle in this 

 cavity showed how they had sunk, by pressure of the external 

 mud, as the eyeball became collapsed by escape of the humours 

 in decomposition. 



Whenever the antecedent forms of an extinct genus of any 

 class are known, the characters of such genus should be com- 

 pared with those of its predecessors in such class, rather than 

 with its successors or with existing forms, in order to gain an 

 insight into its true affinities. 



We derive a truer conception of the affinities of the 

 Ichthyosaurus by comparison with the Labyrinthodonts and 

 other triassic reptiles, as we do of the Plcsioscmriis by com- 

 parison with the muschelkalk Sauroptcryyia, than of either 

 by comparison with modern Lacertians and Crocodilians. It is 

 commonly said that the Iclbtliyo- or the Plcsio-saurus resembles 

 more the lizards in such and such characters, and in a less 

 degree the crocodiles, as in such a character. The truer 

 expression would be that the lizards, which are the predo- 

 minating form of Saurians at the present day, have retained 

 more of the osteological type of the triassic and oolitic reptiles, 

 and that the crocodiles deviate further from them or exhibit 

 a more modified or specialized structure. The posterior 

 position of the nostrils, the small size and position of the 

 palato-pterygoid foramen, are marks of affinity to Plesiosav/rus, 



