142 



EEPTILIA 



CLASS III 



quadra to-jugal, when present, lies above and anterior to the quadrate, 

 connecting it with the jugal. The latter joins the hinder end of the maxilla, 

 and forms with it the inferior border of the orbit. The bony arcade 

 separating the orbits and temporal fossae is formed by either the postorbital 

 alone, or by that bone in connection with the postfrontal and jugal. The 

 prefrontals are usually well developed, and adjoin the frontals on either side ; 

 sometimes they exclude the latter from the superior border of the orbit, and 

 assist the lachrymals in forming its anterior border. The nasal capsule is 

 roofed by the prefrontals and nasals, and bounded anteriorly by the paired or 

 unpaired premaxillae. The latter, together with the maxillae, complete the 

 front rim of the upper jaw. 



The chief modifications of the skull amongst different orders consist in the 

 rearrangement of tl\e bones of the postero-lateral regions into separate bars 

 or arcades, accompanied by the formation of temporal vacuities (Fig. 236). 



Fig. 23) 



Diagram of the cranial roof in a Stegocephalian, various types of Reptiles, and a Bird, showing modifications 

 in the postero-lateral region. A, Mastodonsaurus, 1/15. ]!, Generalised Theromorph or Sauropterygian, passing 

 with slight modification into the Chelonian (sutures dotted to denote inconstancy in fusion of elements). (', 

 Ichthyosaurus, 1/20. D, Generalised Rhynchocephalian, Dinosaurian, Crocodilian, or Ornithosaurian. ]■', 

 Generalised Lacertilian. F, Generalised Bird, fr, Frontal; ,/', Jugal ; /, Lateral temporal vacuity; la. Lachry- 

 mal ; nu\ Maxilla; n. Narial opening; na, Nasal; 0, Orbit; jxi, Parietal; pmx, Premaxilla; prf, Prefrontal; 

 ptf, Post frontal ; pto, Postorbital ; qj, Quadrato-jngal ; qu, Quadrate ; s, Supratemporal vacuity ; sq, Squamosal. 

 Vacuities shaded with vertical lines, cartilage bones dotted (from A. S. Woodward). 



Among Theromorpha, Sauroptcr//(jia, Chelonia, and Ichthyosauria, a supratemporal 

 vacuity is produced by the squamosal being thrust outward to form with the 

 quadrato- jugal and jugal a broad arch along the cheek. A further stage is 

 exhibited by the Rhynchocephalians, Dinosaurs, Crocodiles, and Pterosaurs, 

 where the arrangement of the squamosal is the same, but the broad arch of the 



