Sauropsida: Class Reptilia 501 



an outer plate consisting of dermal bones, mainly the squamosal, 

 jugal, and postorbital. In its simplest form this plate is an uninter- 

 rupted arch extending from the lateral border of the parietal above, 

 down to the series of bones forming the upper jaw. The space thus 

 enclosed (temporal cavity) opens externally in front and behind. It 

 is occupied by the bulky and powerful muscle which closes the mouth 

 by pulling the lower jaw upward. In most reptiles, however, the tem- 

 poral arch, or "arcade," is pierced by one or two large "windows" or 

 fenestrae. These openings doubtless ease the working of the muscle 

 by giving opportunity for bulging when it contracts. When there are 

 two fenestrae in each arcade, one is dorsal to the other and they are 

 separated by a bridge formed by extensions of the squamosal and 

 postorbital bones (Fig. 394). In some reptiles there is only an upper 

 temporal fenestra, the squamosal and postorbital meeting below it. 

 In others there is only a lower fenestra, the squamosal and postorbital 

 meeting above it. Skulls of the anapsid type, having no temporal 

 fenestrae, are found in the most ancient reptiles (cotylosaurs) and in 

 Chelonia (more or less modified in many of them). A skull having only 

 the upper fenestra is called "parapsid." The skulls of lizards and 

 ichthyosaurs have been described as being of the parapsid type, but 

 much doubt exists as to the interpretation of the obscure structure of 

 the temporal regions of these skulls. The diapsid skull, having both 

 upper and lower fenestrae, is found in Rhynchocephalia (Fig. 373), 

 dinosaurs, pterosaurs, and crocodilians. With considerable modifica- 

 tion, it appears in birds. This type was most common in ancient rep- 

 tiles. The synapsid skull, having only the lower fenestra, occurred 

 in the mammal-like reptiles (Theromorpha, Therapsida) and is 

 found, with some modification, in mammals. 



Systematic groups of reptiles (Anapsida, etc.) have been defined 

 on the basis of these differences in the number and position of temporal 

 fenestrae. It is by no means certain, however, that these character- 

 istics, by themselves, are a reliable index to phylogenetic relationships. 



Fig. 393. A Jurassic ichthyosaur, much reduced. (After von Stromer. Courtesy, 

 Romer: "Vertebrate Paleontology," Chicago, University of Chicago Press.) 



