154 



REPTILIA 



CLASS III 



maxillae are all unpaired, as are also the nasals, which are fused with the 

 premaxillae. The supratemporal vacuity is bounded externally by the 



prosquamosal and postorbital, which form a 

 narrow arcade. The squamosal is intercalated 

 between the exocciptal ( = parocciptal, Baur) 

 and the prosquamosal, and sends a slender 

 process inwards between the exoccipital and 

 prootic, extending nearly as far as the brain 

 cavity. The suture between the postorbital 

 and postfrontal is often obliterated. Super- 

 ciliary plates have not been observed, and are 

 often functionally replaced by the horizontally 

 expanded prefrontals. The jugal unites with a 

 descending process of the postfrontal, and these 

 two bones together with the prefrontal com- 

 pletely enclose the orbit. There is a complete 



Bo Opo 



Fig. 243. 



Clidastes propython, Cope. Upper 

 Cretaceous; Kansas. Palatal aspect 

 of skull,! partly restored, (bo, Basi- 

 occipital ; mx, Maxilla; )>l, Palatine; 

 /mix, Premaxilla ; pt, Pterygoid ; ptf, 

 Postorbito- frontal ; opo, Supratem- 

 poral ; ipij, Squamosal ; sph, Basi- 

 spheuoid ; vo, Vomer.) 



Fio. 244. 



Platecarpus coryphaeus, Cope. Upper Cretaceous; Kansas. Pos- 

 terior aspect of skull, 1/4. (bo, Basioccipital ; eo, Exoccipital ; p, 

 Parietal; pe, Prootic; pfo, Postorbito - frontal ; pt, Pterygoid; 7, 

 Quadrate; so, Supraoccipital ; sij, Squamosal; st, Stapes), (after 

 Williston.) 



parietal arch formed by a slender process of the parietal uniting with a 

 process of the squamosal. 



The quadrate (Figs. 245, 246) is relatively large, and movably articulated 

 with the squamosal and prosquamosal. Its antero-superior margin is dilated 

 into a broad thin plate, and its posterior margin is notched or perforated for 

 the auditory meatus. This opening is either partially or completely enclosed 

 posteriorly by the stout suprastapedial process, which is especially character- 

 istic of the Mosasaur quadrate. On the concave inner side of the quadrate, 

 near the upper part of the meatal notch, is a small pit for the lodgment of the 

 outer end of the stapes. The quadrate exhibits wide variation in form and 

 size amongst the different genera. 



The transverse bone, or ectopterygoid, is always small, and joins the 

 pterygoid and jugal only. A slender columella, or epipterygoid, is present in 

 all cases. The mandibular rami were united at the symphysis by ligaments. 

 Somewhat back of the middle of each ramus, between the angular and splenial, 

 and the dentary and surangular, there is a movable articulation permitting of 

 a wide distension of the jaws. 



