76 



VERTEBRATE SKELETON 



postorbital fosste. The- more lateral of these, the infratemporal 

 fossa, is usually bounded on the lateral side by an arcade of squa- 

 mosal, quadratojugal and zygomatic (when the quadratojugal is 

 absent, by the other two) . On the medial side an arcade of squamo- 

 sal and postorbital or postfrontal, separates this fossa from the 

 supratemporal fossa, which is limited medially by parietal and 

 frontal bones. The third, the post-temporal fossa, lies between 

 parietal, supratemporal and occipital bones, the opisthotic and 

 squamosal sometimes entering its boundaries. Occasionally but 

 one of these fossae may be present. On the other hand supra- and 



Fig. 79. — Schema of the formation of vacuities (cross-lined) and arcades in Reptilia 

 (Versluys, '19). A, primitive reptile; B, diapsid type; C, first synapsid type (Thero- 

 morphs); D, second synapsid type, (Sauropterygia and Placodonta); E, Chelonia; F, 

 Ichthyosaurs. The dotted lines, extent of later vacuities. /, frontal; «/, intertemporal; 

 /, lacrimal; m, maxilla; p, parietal; pf, prefrontal; pm, premaxilla; po, postorbital; pof, 

 postfrontal; qj, quadratojugal; sq, squamosal; st, supratemporal; s, zygomatic. 



infratemporal fossae may unite (temporal fossa) by interruption of 

 the upper arcade. Lastly the lower arcade may break, leaving the 

 temporal fossa incomplete, while by interruption of the postorbital 

 bar (postfrontal, postorbital, zygomatic) orbit and temporal fossa 

 are confluent. 



The origin and relations of these fossae have had various explanations, and 

 more than one classification of Tetrapoda has been based upon them. One of 

 these views is illustrated in figure 79. Another view has given some useful 

 terms, as by it groups with but a single (the superior) fossa are known as Synapsi- 

 dans, those with both upper and lower fossse are Diapsidans. 



