REPTILES. 299 



selves are separated by a more or less complete interorbital sep- 

 tum. Correlated with this is the frequent absence of all- and 

 orbitosphenoid ossifications, their places being taken by vertical 

 processes of parietals (turtles) or frontoparietals (snakes) ; 

 while the frontals frequently take no part in roofing in the cra- 

 nial cavity, but are placed above the interorbital septum. 



The membrane bones of the cranium are numerous, the 

 frontals and parietals of the two sides being frequently fused in 

 the median line. Between the parietals in Hatteria and most 



FIG. 295. Lateral view of the skull of Hatteria (Sphenodon}, after Giinther. 

 _//', frontal ; y, jugal ; /, lachrymal; true, maxillare ; n, nasal; oo, opisthotic ; pa, 

 palatine ; pf, prefrontal ; pm, premaxillary ; po, postorbital ; pof, postfrontal ; //, 

 pterygoid; q, quadrate; </j, quadratojugal ; st/, squamosal. The supra- and infra- 

 temporal fossae shown above and below the postorbital-squamosal arch. 



lizards, as well as in many fossil forms, occurs a well-marked in- 

 terparietal foramen, connected, at least in the living forms, with 

 a well-developed parietal eye. The ethmoid region is covered 

 by the paired nasals, while in lizards they are covered by paired 

 vomers. In other forms the vomer may be median and un- 

 paired. Prefrontals are almost always present, while postfrontals 

 usually occur ; and in lizards, crocodiles, and many extinct forms 

 lachrymals are present. In many fossils and in lizards a supra- 

 temporal bone occurs between squamosal and quadrate ; while in 

 lizards, snakes, crocodiles, and ichthyosaurs an os transversum 

 connects the maxillary with the pterygoid. In dinosaurs a 

 rostral bone may occur in front of the premaxilla. 



The lower jaw is always suspended from the quadrate; and 



