112 University of Kansas Geological Survey. 



Mosasaurus. The postorbito-frontals in Mosasaurus horridus 

 extend forwards to touch the posterior end of the prefrontal, 

 leaving a triangular notch in the roof of the orbit. The pos- 

 terior process is wedge-shaped, and, posteriorly, instead of 

 being inserted in a groove in the squamosal, it divides that 

 bone in two, one part of which lies on the inner and the other 

 on the outer side below. 



Platecarpus. PI. xxiv, f. 4. In this genus the postfrontal 

 touches, or nearly touches, the prefrontal, instead of being 

 separated by a considerable space as in Clidastes. The jugal 

 process is longer and natter. The posterior projection, lodged 

 in a groove on the upper part of the prosquamosal, extends 

 back to beyond the anterior end of the articulation for the 

 quadrate. The bone does not articulate with the squamosal 

 at all, and the prosquamosal reaches as far forward as the de- 

 scending process for the jugal. 



Tylosaurus. In Tylosaurus these elements are nearly as in 

 Platecarpus, except that the bone extends forward, quite to the 

 prefrontal, forming a very narrow margin to the orbit posteri- 

 orly. The jugal process is rather shorter, and the articular 

 surface for the prosquamosal reaches quite to that for the jugal, 

 the two bones touching each other, which does not occur in 

 Clidastes. The bone underlaps the frontal broadly, forming a 

 subtriangular surface. 



Jugal. 



Clidastes. The jugal is a slender, curved rod, with an ar- 

 ticular surface at either extremity. It is nearly circular in 

 cross-section posteriorly and flattened anteriorly. The curva- 

 ture begins near the middle, and is nearly regular. The ante- 

 rior half is only slightly concave on the upper border. The 

 excavation for the maxilla is on the inferior inner side, and 

 reaches nearly a third of the length of the bone. At the ante- 

 rior dilated extremity the bone articulates with the lachrymal 

 above. The posterior extremity is somewhat thickened, though 

 not much dilated, and unites with the short jugal process of the 

 postfrontal. The bone has no tubercular process on the posterior 



