Page:The Osteology of the Reptiles.pdf/30

12 Openings through the skull roof, back of the orbits, are characteristic 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. 33 ) from the parietal (Fig. 53 ). The lower or lateral temporal opening appeared primitively (Figs. 33, 53 ) 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 ). Either the upper or the lower opening may occur independently, or both together. In the Cynodontia (Fig. 45) and some other Theriodontia, with a lower temporal opening (Figs. 44, 45 ), 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 ), 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 Placodontia (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, lies 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