1 62 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



have been named ; it usually enters into the composition of the 

 anterior wall of the orbit, but it may be forced forward by 

 the prefrontal. 



FlG. 171. Ventral and side views of the skull of Hatteria (Sphenodon*), after 

 Giinther. bo, basioccipital ; bs, basisphenoid ; eo, exoccipital ; fr, frontal ; j, jugal ; 

 /, lachrymal ; mx, maxillare ; n, nasal ; oo, opisthotic ; pa, palatine ; pf, prefrontal ; 

 pni, premaxillary ; po, postorbital ; pof, postfrontal ; pt, pterygoid ; q, quadrate; qj, 

 quadratojugal ; sq, squamosal ; z>, vomer. 



In the elasmobranchs the upper jaw is formed by the ptery- 

 goquadrate cartilage, but in all higher forms other elements 

 usurp these functions. In front there are a pair of premaxilla- 

 ries already mentioned, and behind these a pair of maxillaries 



