Owen to the present. The large temporal vacuity is admittedly the upper one, bounded on the inner side by the parietal, on the outer by the postfrontal and the so-called supratemporal. There is no lateral foramen, and it is quite improbable that a preëxistent one was later closed by the encroachment of the orbit. This region, as in the primitive skull, has five bones. About three, the postfrontal, postorbital, and quadratojugal, there can be no question of identity. And unless we accept the wholly improbable theory that new bones have been developed in the temporal region of the ichthyosaurs, the other two must be homologized with the supratemporal, or tabular, and the squamosal.
Fig. 51. Ichthyosaur skull: Baptanodon (Ophthalmosaurus), from the rear. After Gilmore. Ang, angular; bs, basisphenoid; d, dentary; f, frontal.
The supratemporal bone was the first to be lost in the primitive skull, and there is no certain evidence yet forthcoming that it was retained in any reptiles after the cotylosaurs. If, however, the supratemporal was persistent in the ichthyosaurs instead of the tabular, by no possibility can it be the bone on the outer side of the squamosal, as some recent writers assert, as a comparison of the cotylosaur skull will make evident. The outer bone, sometimes obsolete in ichthyosaurs, must be the squamosal. The upper, posterior bone completing the upper border of the temporal vacuity, the author prefers to believe is the tabular and not the supratemporal, and doubtless is homologous with the bone so recognized in the skull of the Squamata. We cannot conceive of its being anything else, having as it does the same relations with paroccipital, parietal, and quadrate, rarely in the mosasaurs extending forward to articulate with the postorbital.
The Skull of the Protorosauria
(Figs. 52, 53 c–e)
In the order here provisionally called the Protorosauria the skull is completely known in none, but best in Araeoscelis, the oldest cer-
