Williston.] Mosasaurs. 117 



the bone. On either side, the bone sends down a broad, deep 

 wing to form the upper part of the sides of the brain-case. At 

 the most inferior part the border is projected into a thin, tri- 

 angular process, the tip of which shows sutural roughening ; 

 their posterior margins are thin, with an S-shaped undulation. 

 From the sides this thin margin shows an obtuse angle near 

 the middle, the upper part for union with the supraoccipital, 

 the lower for the petrosals, or rather for the cartilage that com- 

 pletes the union between these two bones. The thin margin 

 superiorly, is inserted in the groove of the supraoccipital, as 

 described below. Between the upper extremities of these thin 

 margins the under surface of the bone is more or less rough- 

 ened, or with spinous sutural projections for attachment to the 

 crest of the supraoccipital. The posterior processes are thin, 

 flattened, and arch outwards, backwards and downwards to 

 unite with the long process of the squamosal by a long suture 

 on the under side. The superior surface between the crests has 

 nearly parallel sides. The bone articulates with the frontal, 

 postfrontal, petrosal, squamosal, and supraoccipital. 



Mosasaurus . The superior surface of the parietal continues 

 back more narrowly than in Clidastes, with a divaricated proc- 

 ess on either side of the posterior notch. 



Platecarpus. Pl.xxvi,f. 1; pi. lxiii, f. 1. The parietal fits into 

 a broad emargination of the frontal. From the posterior angles 

 of this emargination, the sharp borders of the superior surface, 

 or the parietal crests, run nearly straight to an apex a little be- 

 yond the middle of the bone, a character peculiar to the genus. 

 The moderately large parietal foramen is situated a little back 

 of the line of the suture, and usually wholly within the parietal 

 bone. The lateral processes, forming the anterior boundary of 

 the supratemporal fossa, reach out transversely to unite with 

 the postfrontal a little within the angles of the frontal bone. 

 The bone is narrowest opposite the apex of the superior surface. 



Tylosaurus. The parietal in Tylosaurus is characteristic. Its 

 upper flat surface has its sharp, lateral crests convergent to be- 

 yond the middle, and then parallel or gently divergent, the pos- 



