An ichthyosaur skull from the Mowry Shale of Wyoming--the only good ichthyosaur skull so far known from the Cretaceous--is described and figured. Study of the well-preserved temporal region leads to the conclusion that the dermal element usually described in ichthyosaur skulls above the quadratojugal is nonexistent; that, hence, the large element bounding the temporal opening is the true squamosal, and the opening is thus of a typical euryapsid pattern. Illustrations of a Liassic ichthyosaur prepared by D. M. S. Watson are appended.
