110 University of Kansas Geological Survey. 



concavity of the bone. On the under side there are two promi- 

 nent ridges, bounding the cavity for the olfactory lobes. On 

 the anterior part this cavity is separated into two by a median 

 ridge ; nearly opposite the posterior extremity of the maxilla 

 the ridges bounding the cavity approach each other, the exca- 

 vation continuing as a narrow, deep groove to about opposite 

 the middle of the orbital margin. At the outer side of these 

 ridges the bone is excavated for the sutural union of the pre- 

 frontals. At the beginning of the narial opening the bone rap- 

 idly narrows and is thinned for union with the nasal. In this 

 part the bone is imperfectly united in the middle. The nasals 

 reach no further than the posterior end of the narial openings. 

 The bone articulates with the parietal, rjostorbital, prefrontal, 

 and nasal. 



Mosasaurus horridus. The frontal bone in Mosasaurus is much 

 broader than in Clidastes. Posteriorly it has two slender proc- 

 esses inclosing the narrow anterior projection of the parietal, in 

 which is the pineal foramen. The sides are nearly straight, and 

 gently convergent to a strong and sharp emargination for the 

 prefrontal anteriorly. Its connection with the nasal cannot be 

 made out with certainty, but it is evidently not posterior to the 

 narial openings. The bone is nearly plane, sloping gently, and 

 with a low, obtuse carina in the middle anteriorly. 



Platecarpus. PI. xvi, ff. 2, 3. The frontal bone is broader 

 than in Clidastes. Posteriorly it is broadly and deeply emargi- 

 nate for the parietal, outside of which the margins are nearly 

 straight to the angles. From near the angles the lateral mar- 

 gins are concave for nearly half their distance to the nares, 

 somewhat thickened on the orbit, which is thickened for a short 

 distance, as seen from above. Beyond, the sides are irregular 

 and gently convex to unite with the prefrontals. From a little 

 back of the middle of the bone there is a thin, sharp, median 

 carina, highest in its middle portion ; on either side of the ca- 

 rina the bone is shallowly concave. On either side in front 

 there is a small, tooth-like process underlying the posterior 

 process of the maxilla, the notch between it and the inner bor- 



