THE TEMPORAL ARCHES OF THE REPTILIA. 189 



troversy and differences of opinion, differences which are by no 

 means yet settled. The single arch present (Fig. 7) has been 

 variously considered to be the lower arch, the upper arch, or a 

 compound of both arches. It is composed (when present) of 

 three bones ; an anterior one, the postorbital, extending back- 

 ward to unite with a middle one ; the middle one joining the 

 postorbital in front, the posterior one and often the parietal 

 behind, and more or less broadly articulating with the upper end 

 of the quadrate; and a posterior element, joining the parietal 

 internally, the middle element anteriorly, and united with the 

 petrosal and exoccipital below, and usually also articulating with 

 the head of the quadrate. The middle bone has been called the 

 temporal, squamosal, quadratojugal, prosquamosal, supratem- 

 poral, zygomatic, supramastoid, paraquadrate ; the posterior ele- 

 ment, the mastoid, squamosal, supratemporal, supramastoid, and 

 paroccipital. At present most writers, following Baur, 1 call the 

 middle element the prosquamosal, and the posterior one the 

 squamosal, though Woodward 2 applies the name squamosal to 

 the middle element, and supratemporal (prosquamosal) to the 

 posterior one. 



It is now generally assumed that the Squamata have descended 

 from the rhynchocephaloid reptiles. We have seen that in the 

 early diapsid reptiles the prosquamosal is not articulated between 

 the squamosal and the postorbital, but forms a part of the lower 

 arch between the quadratojugal and the jugal. Here, however, 

 it is assumed that the lower arch of the rhynchocephs has dis- 

 appeared, that the quadratojugal is lost, but that the bone artic- 

 ulating with the quadratojugal between it and the jugal has been 

 transferred to the upper arch, a position unknown in any other 

 reptile, recent or extinct, to bound the outer part of the supra- 

 temporal vacuity. One could with as much reason call it the 

 quadratojugal with former authors and with Baur 3 when he 

 believed that the Squamata were not closely related to the rhyn- 

 chocephalians, and that the arch of the Squamata was formed, 

 not by the loss of the lower arch, but by the fusion and attenu- 



1 Anat. Anzeiger^ X., p. 328, 1895. 



2 " Vertebrate Paleontology," p. 143, Fig. E; p. 192, Fig. A, 1898. 



3 Journ. Morphology, III., 473, 1889. 



