AVilliston.] Mosasaurs. 119 



is sometimes found, and in all probability it occurs in Clidastes. 

 The lateral processes, the basipterygoid, are much shorter than 

 in Varanus, and evidently are completed by cartilage, as they 

 do not extend *to the bottom of the notch for their reception in 

 the pterygoids in the articulated skull. The posterior, basi- 

 occipital processes are squamous, underlying and closely united 

 with the hypapophyses of the basioccipital, reaching nearly to 

 their extremity. From above, the narrow, concave brain cavity 

 is seen lying between the oblique, broad, sutural surfaces for 

 the petrosal. On either side, under these overhanging sutu- 

 ral projections, there is a deep longitudinal groove, at the 

 bottom of which, in front and behind, is the opening to the 

 longitudinal canal. From the anterior opening of the canal a 

 groove is continued anteriorly to near the tip of the presphenoid 

 process. In front of the pituitary fossa the cerebral surface 

 is narrowed to a slender groove, continued to the tip of the 

 presphenoid process. The bone articulates posteriorly with 

 the basioccipital, superiorly with the petrosals, and anteriorly 

 with the pterygoids, the latter of course non-sutural in character. 



Basioccipital. 



Clidastes velox. The occipital condyle is moderately separated 

 from the basioccipital processes by a distinct neck, and is chiefly 

 formed by this bone. The neural surface is narrow, of moder- 

 ate depth, with a depression near the middle, the " fossa medi- 

 ana." The hypapophysial processes are stout, trihedral, directed 

 downward and outward, and are overlapped on their whole 

 anterior surface by the thinned posterior process of the basi- 

 sphenoid. There is no median canal in any specimens of this or 

 the other genera in the University collection. The bone articu- 

 lates above w^ith the exoccipitals, which extend downward on 

 the outer side nearly to the extremity of the basal processes. 

 In front the bone articulates with the basisphenoid, the anterior 

 broadly rounded border of the inferior processes fitting into a 

 depression on the posterior part of the basisphenoid. On the 

 upper anterior angles there is a small surface for articulation 

 with the petrosals. 



