108 University of Kansas Geological Survey 



short part of the. free border of the nares, anterior to which the 

 thin expansion is overlapped by a flattened process of the max- 

 illary. The maxilla also overlaps the outer part of the bone on 

 the anterior third. The horizontal part is considerably thickened 

 posteriorly, with the under side of the hind border beveled and 

 slightly concave. A broad, flattened process at the posterior 

 interior part underlaps the frontal, being received in a pit or de- 

 pression of that bone. A thick, curved, wing-like process is 

 sent down on the outer side, curving inward to form the very 

 convex anterior border of this orbit, and articulating below with 

 the jugal, lachrymal, and, on the inferior surface, with the pos- 

 terior end of the palatine. It articulates with the frontal on 

 the inner side, the maxilla in front and on the side anteriorly, 

 the lachrymal and jugal behind, and the palatines below. 



Mosasaurus horridus. The prefrontal of Mosasaurus is quite 

 as peculiar as that of Clidastes. The horizontal part, or wing, 

 over the anterior part of the orbit, is semicircular in shape, 

 with lightly crenulated edges. In front of this there is an 

 emargination or groove with rounded border connecting the 

 superior and lateral faces. This semicircular supraciliary plate 

 served the same purpose as the posterior flattened part in the 

 Clidastes bone. Were it removed the bone would not be unlike 

 what it is in the two following genera. Between this supra- 

 ciliary plate and the postfrontal there is a rather sharp trian- 

 gular notch, evidently filled in with membrane in life. The 

 bone touches the postfrontal behind. 



Platecarpus. PI. xxv, f. 3. The prefrontal, as seen in the ar- 

 ticulated skull of Platecarpus, sends a long, slender process on 

 the inner margin of the maxilla as far as the posterior end of 

 the anterior emargination, its thin narial border being straight 

 or gently concave. Posteriorly it has a thickened, irregular, 

 very narrow horizontal projection along the frontal orbit, corre- 

 sponding to the semicircular plate of Mosasaurus or the broad 

 one of Clidastes. Behind this it reaches back nearly or quite as 

 far as the postfrontal, though scarcely visible from above. In- 

 feriorly it sends down a much smaller process for union with 

 the palatines. 



