"Williston.] Mosasaurs. 129 



in Clidastes. The inferior anterior angle is acute and the border 

 for a short distance is thin and sharp. The anterior surface is 

 only moderately convex longitudinally, but has a distinct chan- 

 nel, concave from side to side. The alar process of the upper 

 articular surface is not as long or slender as in Clidastes; that 

 for the suprastapedial process broad and not constricted. This 

 process is long, reaching much below the middle of the bone, 

 and arches far backward. It is stout, only a little expanded 

 distally, and incloses a large, broad notch between it and the 

 body. Its inner side above is beveled or excavated much as in 

 Brachysaurus , leaving a free rounded border next the opening. 

 About midway in the process there is a rather large, rounded, 

 smooth articular surface on the inner side, posteriorly. The 

 ala is very large, broad, and thin. Its border is continuous 

 from the end of the suprastapedial process to the rugosity below 

 the process, and is everywhere thin and prominent, and nearly 

 all on one plane. At the lower part, there is a much deeper 

 concavity than in the other forms, behind and above the very 

 prominent ridge or plate that continues the border to the ru- 

 gosity. This rugosity is in the shape of a small tubercle. The 

 inferior articular surface is small — very small in proportion to 

 the size of the bone — and has a prominent articular projection 

 in the middle in front. The bone is very large in proportion to 

 the size of the skull, as compared with that of the other genera. 



Tylosaurus . Pis. lx, lxi. As is the case with Clidastes and 

 Platecarpus, the quadrate of Tylosaurus is very characteristic 

 of the genus, resembling, however, that of Clidastes more than 

 that of Platecarpus. Anteriorly the shape is much like that 

 of C. tortor, the inner margin less concave, the outer less con- 

 vex, the shape being more like that of a parallelogram, with 

 the two articular ends beveled outwardly. The surface is less 

 convex from above downward, and there is a distinct longi- 

 tudinal channel, the surface transversly in the middle not 

 being plane as in C. tortor. The internal face is much* like that 

 of C. tortor, the concavity and sinuosity less. The stapedial 

 pit is moderately oblique and extends for about half its length 



