AVilliston.] Mosasaurs. 109 



Tylosaurus. In Tylosaurus the superior surface of the pre- 

 frontal is elongate triangular in shape. The anterior end is 

 overlapped broadly by the broad, tongue-like process of the 

 maxillary, which may even reach to the frontal, wholly exclud- 

 ing the prefrontal from participation in the nares. Posteriorly 

 it reaches, narrowly, quite to the anterior prolongation of the 

 postfrontal. Outwardly it reaches downwardly, broadly, to the 

 thin margin of the maxilla, leaving a triangular space pos- 

 teriorly in which the lachrymal is articulated. In front of the 

 orbit the nearly vertical surface turns inward, broadly, as in the 

 preceding, to the margin of orbits. In the horizontal develop- 

 ment of the posterior part of the prefrontal, the greatest is that 

 of Clidastes, the least of Tylosaurus, with Mosasaurus and Plate- 

 carpus respectively intermediate. 



Lachrymal. 



The lachrymal bone is present in the different genera, but 

 seems to differ slightly. It is least distorted in a specimen 

 of Clidastes, where it is a small, somewhat irregular, pointed 

 bone, with an articular surface along the sides, and at the 

 larger extremity is grooved along one side ; it is smooth above, 

 and is slightly roughened on the outer side. It is attached in 

 the interval between the jugal and the prefrontal, and articu- 

 lates in part with the maxilla. It enters into the lower part of 

 the anterior border of the orbit. 



Frontal. 



The frontal bone in Clidastes is elongate and slender in com- 

 parison with that of the other genera. The lateral borders have 

 a long and deep concavity from near the posterior angles of the 

 bone to near the anterior third, whence the sides approach each 

 other more rapidly, or wedge-like, to near the posterior ends of 

 the narial openings. The posterior border is transverse, with 

 an undulatory or zigzag outline, the middle not being apprecia- 

 bly emarginated for the parietal bone. The upper surface is 

 flattened, gently convex in the middle, and with a low, rather 

 obtuse median carina on the anterior third. The free orbital 

 border is short, obtusely edged, and forms a part of the lateral 



