12 THE OSTEOLOGY OF THE REPTILES 



Openings through the skull roof,^ back of the orbits, are character- 

 istic of all reptiles save the Cotylosauria (Figs. 2, 4, 19, 22) and 

 Chelonia. The upper opening, the supratemporal, arose primitively 

 by the separation of the postorbito-squamosal bar (Fig. ^3 a) from 

 the parietal (Fig. 53 c). The lower or lateral temporal opening ap- 

 peared primitively (Figs. S3, 53 a) between the squamosal and the 

 jugal. It is bounded above by the postorbito-squamosal arch, below 

 by the jugal, to which was added, in some of the double-arched 

 forms, the quadratojugal (Figs. 62, 64, 65, 70 A, b). Either the 

 upper or the lower opening may occur independently, or both to- 

 gether. In the Cynodontia (Fig. 45) and some other Theriodontia, 

 with a lower temporal opening (Figs. 44 f, 45 d), the squamosal may 

 fail to meet the postorbital above the opening, permitting the 

 parietal to form the upper boundary in part; and this is the condition 

 in mammals. In not a few of the Therapsida, the Dinocephalia 

 especially (Fig. 44 b), the jugal is excluded from the lower margin 

 by the union of the squamosal and postorbital. 



All known forms of the Sauropterygia (Figs. 46, 47, 48) and Placo- 

 dontia (Fig. 49) have the single opening bounded below by the 

 squamosal and postorbital, above by the sides of the parietal, that is, 

 it is like the upper one of those reptiles with two temporal openings. 

 It is usually considered to be what it really appears to be, the upper 

 temporal opening only; and its certain nature will not be determined 

 until more is known of their terrestrial antecedents. 



The intertemporal vacuity. The single temporal opening of the 

 Squamata (Figs. 55, 54), when complete, the Ichthyosauria (Fig. 

 50), and certain other forms here grouped under the Parapsida, Hes 

 between the postorbito-squamosal arch and the parietal, but has, in 

 most if not all, an additional bone helping to form its posterior or 

 outer border, the supratemporal or tabular, for there is doubt as to 

 its real homology. (See pages 61-69 below.) 



The post-temporal opening is situated on the occipital aspect of the 

 skull, a vacuity between the parietal, or parietal and squamosal, and 

 the paroccipital on each side. It is present in some Cotylosauria 



1 [In addition to the openings noted by the author, paired subtemporal openings occur 

 in all reptiles in the palatal aspect of the skull; they are bounded medially by the 

 pterygoids and the basis cranii, laterally by the lower temporal bar, if present, or by the 

 dermal covering of the temporal region, as in Squamata. — Ed.] 



