ICHTHYOPTEKYGIA 205 



occipito-parietal vacuities are larger than in Crocodilia, smaller 

 than in Lacertilia ; they are bounded internally by the basi-, 

 ex-, and super-occipitals, externally by the parietal and mastoid. 

 The auditory apertures are bounded by the tympanic and 

 squamosal. The tympanic takes a greater share in the forma- 

 tion of the " meatus auditorius " in many lizards ; in crocodiles 

 it is restricted to that which it takes in Ichthyosaurus. 



The orbit is most remarkable in the Ichthyosaurus, amongst 

 reptiles, both for its large proportional size and its posterior 

 position ; in the former character it resembles that in the 

 lizards, in the latter that in the crocodiles. It is formed by 

 the pre- and post-frontals above, by the lacrymal in front, by 

 the post-orbital behind, and by the peculiar 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 aperture, 

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

 nasal, maxillary, and premaxillary 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 premaxillary. 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 fossa 1 are bounded above 



