Page:The Osteology of the Reptiles.pdf/89

Rh pit on the upper side of the pterygoids and extending to or toward the parietals.

The frontals usually and the parietals always are fused in the midline (Fig. 55 ).

The parietal foramen, usually present, is absent in many terrestrial lizards and in the Rhiptoglossa. The frontals and parietals may be either paired or unpaired. The frontals in the Varanidae, Helodermatidae, and some others have descending processes of the frontals which meet in the middle below, enclosing a rhinencephalic chamber, very much like the primitive one of the early reptiles.

The brain-case of lizards, as of other reptiles, is formed by the supraoccipital, exoccipitals, paroccipitals, basioccipital, basisphenoid, proötics, and postoptics, but is more or less membranous in front on the sides. The postoptics (Fig. 55, al) are small ossifications in the wall membrane, usually lost in maceration. In the Amphisbaenia (Fig. 56 ) and Mosasauria the sides of the parietals are partially decurved, forming incomplete cerebral walls, but they do not reach, as in the snakes (Fig. 59 ), to the basisphenoid.



The mandibles (Figs. 55, 58) are composed of the dentary coronoid, surangular, articular, angular, and splenial, with a long fused prearticular, which in the mosasaurs is more or less ensheathed by the union of the coronoid and angular, strengthening the peculiar