Williston.] Mosasaurs. Ill 



der forming the posterior angle of the nares. This process is 

 wholly wanting in Clidastes. On the under side the prefrontals 

 are more widely separated and the olfactory groove more grad- 

 ually widened. The bone is considerably wider at posterior end. 



Brachysaurus overtoni. The frontal bone in this species is re- 

 markably broad and stout. 



Tylosaurus proriger. In Tylosaurus, the frontal is broader 

 relatively than in Platecarpus ; the sides above the orbits are 

 nearly straight and parallel, and wholly excluded from the 

 orbits. From the posterior third the sides gradually converge 

 in nearly a straight line to the tooth-like process at the pos- 

 terior end of the nares. In front of the middle the bone is con- 

 vex in the middle part but is not carinate. Anteriorly the two 

 halves of the bone are unossified, the division represented by a 

 median groove above. Posteriorly the deep median emargina- 

 tion for the parietal is wanting. In shape the bone is more 

 nearly triangular than in the preceding species. On the under 

 side the prefrontals are more narrowly separated than in Plate- 

 car pus. 



Postfronto-orbitals. 



Clidastes. The postfrontal and orbital are closely united, 

 without trace of suture. It extends along the outer border of 

 the frontal for a short distance, its outer border curving out- 

 ward and downward to the jugal process. The stout, thick- 

 ened process for lateral union with the parietal extends only a 

 little inwards, forming the outer anterior angle of the supra- 

 temporal fossa. The jugal process is broad and flat and is 

 directed nearly downwards. Back of the process the bone nar- 

 rows to a flattened oval shape in cross-section, the under side 

 of which is inserted into a groove in the prosquamosal. The 

 bone extends nearly to the articular surface for the quadrate. 

 The broad squamosal plate underlies the frontal, articulating 

 for nearly a third of its width. It articulates with the squa- 

 mosal (sometimes), the prosquamosal, parietal, jugal, and 

 frontal. 



