ICHTHYOPTERYGIA. 225 



in front, by the post-orbital behind, and by the peculiarly long 

 and slender malar bar below. In crocodiles and in most lizards 

 the frontal enters into the formation of the orbits, and in lizards 

 the maxillary also. The nostril is a longish triangular aper- 

 ture, with the narrow base behind ; it is bounded by the 

 lachrymal, nasal, maxillary, and pre-m axillary bones. It is 

 proportionally larger than in the Plesiosaurus, and is distant 

 from the orbit about half its own long diameter. Like the 

 orbit, the plane of its outlet is vertical. 



The pterygo-palatine vacuities are very long and narrow, 

 broadest behind, where they are bounded, as in lizards, by the 

 anterior concavities of the basi-sphenoid, and gradually narrow- 

 ing to a point close to the palatine nostrils. These are smaller 

 than in most lizards, and are circumscribed by the palatines, 

 ecto-pterygoid, maxillary, and prern axillary. The pterygo- 

 malar fissures are the lower outlets of the temporal fossae; 

 their sudden posterior breadth, due to the emargination of the 

 pterygoid, relates to the passage of the muscles for attachment 

 to the lower jaw. The parietal foramen is bounded by both 

 parietals and frontals ; its presence is a mark of labyrinthodont 

 and lacertian affinities ; its formation is like that in Iguana 

 and Rhynchocephalus. The temporal fossse are bounded above 

 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 comparing the jaws of the Ichthyosaurus tenuirostris 

 with those of the gangetic Gharrial, an equal degree of strength 

 and of alveolar border for teeth result from two very different 

 proportions in which the maxillary and premaxillary bones 

 are combined together to form the upper jaw. The prolonga- 

 tion of the snout has evidently no relation to this difference ; 

 and we are accordingly led to look for some other explanation 

 of the disproportionate development of the premaxillaries in 



Q 



