Ahr. 1903. North American Plesiosaurs — Williston. 15 



quence, the fragility of the skull was such that it was not deemed 

 prudent to mount it with the remainder of the skeleton. A model of 

 it was therefore made, based upon my drawings and studies, and 

 which, I think, represents the skull very nearly as it must have been 

 during life. Its width in all parts may not have been accurately 

 determined, but the discrepancies from the reality can not be great. 



The premaxillaries are separated from each other distinctly by 

 suture, the long facial processes apparently lying in contact with each 

 other without close union. The suture separating them from the 

 maxilla begins just back of the sixth tooth ; it curves upward and 

 backward for a short distance, and then runs parallel with the upper 

 border as far back as the narial opening, whence the margin runs more 

 obliquely to the tip of the processes above the middle of the orbit. 

 Each premaxilla bears six teeth, which are among the largest of the 

 jaws, and are all of nearly uniform size, the first one curved forward. 

 The facial process is slender, flattened on its opposing, sutural sur- 

 face, and with its external, convex surface distinctly striated longi- 

 tudinally. The dentigerous portion is convex, pitted toward the 

 anterior part, and about twenty- five millimeters in height, opposite 

 the last tooth. The relations of the bone on the palatal surface can 

 not be determined. 



The maxilla are long and narrow on the facial surface, and 

 very narrow on the palatal surface, at least posteriorly. They bear 

 twenty teeth on each side, the first ten or eleven of which are 

 of nearly equal size, and scarcely smaller than those of the pre- 

 maxilla?. The posterior ten teeth are crowded, occupying a 

 space less than one-half that of the preceding ten, and they are 

 smaller. The greatest width of the maxilla on the facial surface 

 —about twenty-five millimeters — is at about seventy millimeters in 

 front of the orbit, whence the bone narrows to a width of ten milli- 

 meters below the anterior border of the x orbit. Below the orbit, the 

 bone extends as a narrow bar, becoming slightly narrower posteriorly, 

 before the beginning of the jugal suture. Beyond this, it flattens 

 posteriorly to near its extremity, which is about midway of the tem- 

 poral bar, and one hundred millimeters beyond the last tooth. 



There are twenty-five or twenty-six teeth in each jaw. They are 

 inserted by a long fang, the pulp cavity of which occupies more than 

 one-third of the diameter, extending a short distance into the crown. 

 In the largest teeth, the crown is about twenty millimeters in length, 

 with a diameter at the base of six millimeters. The crown is rela- 

 tively slender, strongly convex anteriorly, sharply conical, and with 

 slender, delicate, longitudinal stiia 1 , except on the outer, anterior 



