10 THE MORPHOLOGY OF THE SKULL 



stiicke vertreten zu sein." He then cites Lortet's description of the skull ('93) as 

 incorrect, and Boulenger's remarks on Lortet's description ('93) to support his own 

 contention as to the separate nature of the elements. Boulenger said " The bones 

 described as the posterior portions of the parietals appear to be the supratemporals 

 (=prosquaraosals), distinct from the squamosals." 



In the Ichthyosaurs the two bones are always separate. 



In the Dinosaurs, Ph j/tosaurs, Crocodilia and Pterosaurs tliere is one less element in 

 the temporal complex ; the absent bone belongs to the lower arch, and, judging from 

 its relations, could be either the quadrato-jugal or the prosquamosal ; that it is the 

 latter is shown by the presence of the quadrate foramen, for it is hardly possible that 

 such a fenestra as the quadrate foramen, carrying no vessels, should survive a series of 

 changes involving the disappearance of the quadrato-jugal and the assumption of its 

 position by the prosquamosal. If the above reasoning is correct the foramen quad- 

 ratum assumes a considerable morphological importance, as it marks definitely the 

 posterior bone of the lower arch as the quadrato-jugal. From a consideration of the 

 position of the quadrato-jugal in the Pelycosauria and Sphenodon and a comparison 

 with the position of the same bone in the Crocodilia, Dinosauria and Pterosauria it is 

 easily seen that the forward growth of the quadrato-jugal to unite with the jugal may 

 have pushed up the prosquamosal and excluded it from the lower arch. In the 

 Dinosauria in general, and especially in the Theropodous Dinosaurs, which are the 

 most primitive, and very similar in most points of skull structure to the Pelycosaurs 

 (the Theropodous Dinosaurs are the only ones which possess the quadrate foramen), 

 we find the same sort of an anterior process of the sqvamosal as occurs in Sphenodon. 

 The steps seem perfect from one condition in the Pelycosauria to the other in the 

 Sphenodon and Theropodous Dinosaurs. 



In the Dinosauria where the quadrate foramen is missing, the Saurojwda and Pre- 

 dentata, the Crocodilia and Pterosauria it is safe to assume that the same bone has 

 disappeared as in the forms where the steps can be traced. 



Although the present specimens give no positive evidence concerning the disap- 

 pearance of the lower arch in the Squamata it suggests very forcibly one thought. 

 The foramen quadratum is in its inception in the Pelycosauria (it does not occur in 

 the Cotylosauria or in the primitive Pelycosaurians, Diopexis (Case, '03') and is much 

 larger in Sphenodon ; it seems possible that the same process of fenestration Avhich 

 developed the superior and inferior temporal vacuities may have increased the size of 

 the foramen (juadratum after the exclusion of the prosquamosal from the lower arch, 

 until the quadrato-jugal was loosened from the quadrate and disappeared in the liga- 

 ment that represents the inferior arch in the Lacertilia. 



