40 MOSASAURUS. 



of the lower jaw, obtained by Dr. C. Thompson from the Marl of Freehold, Mon- 

 mouth County, N. J. llestored to their proper place the specimens correspond in 

 size, shape, and number of insertions for teeth with the portion of a jaw of the 

 Maestricht Monitor, figured in Plate VI, of the Historic de la Montague de St. 

 Pierre. The fragments compose twelve inches of the right dental bone, repref^nted 

 in Fig. 13, Plate XI, and sixteen of the left one. The latter presents a depth of 

 four inches gradually decreasing to the anterior end where it measures about two 

 inches. In their length the bones are remarkably straight, and their symphysial 

 extremity, without arching inward, is obtusely rounded. The outer surface and 

 base are convex. The former, half way between the latter and the alveolar border, 

 exhibits a longitudinal series of large elliptical foramina directed obliquely forward 

 and outward from the dental canal. Nearer the base approaching the symphysis, 

 and also in the latter position, there exist a number of similar foramina. The left 

 dental specimen has accommodated nine teeth, which are lost, except the fifth tooth 

 and the fang of the first. The right dental specimen accommodated seven teeth, 

 which are lost, except the fangs of the anterior three. The fang» and alveoli in 

 the specimens are directed obliquely forward and upward with a feeble curvature. 

 The angle of inclination of the fangs successively increases from behind forward. 

 In the fifth tooth it is about 50° ; in the first one about 30°. 



The fifth tooth remaining in the left dental bone, has its croAvn mutilated, but a 

 portion of the inner surface being preserved, exhibits a number of well-defined planes. 

 The fang is about three inches in length, and exhibits a large excavation postero- 

 internally for a successor. 



In the right dental specimen the fang of the first tooth, two and a quarter 

 inches long, presents a small concavity postero-internally, just below the alveolar 

 border, for a successor. The fang of the second tooth is about one-third excavated, 

 and the excavation at base impresses the front of the fang of the third tooth about 

 its middle. Postero-internally the third fang presents a small excavation for its 

 own successor. 



Fig. 11, Plate VIII, represents a basi-sphenoid bone of Mosasaurus, from the first 

 marl bed of Holmdale, ]\Ionmouth County, loaned to me by Prof. Cook. The 

 specimen, a fragment of a huge skull, measures eight inches in length and six 

 inches in breadth at the posterior diverging processes, which abut against the basi- 

 occipital bone. 



The basi-occipital bone, from Mississippi, part of the collection of Dr. Spillman, 

 previously mentioned, is two inches long in the median line, and of ten lines greater 

 width at the anterior diverging processes. The latter are separated by a wide con- 

 cavity from each other and from the occipital condyle. They terminate in a reni- 

 form convexity ; in front sustain the angularly divergent processes of the basi- 

 sphenoid bone, and posteriorly support in part the lateral occipitals. The condj'le, 

 someAvhat mutilated, in its entire condition has measured about two and a quarter 

 inches in transverse diameter, and a little over an inch vertically. The portion 

 contributed by the lateral occipital, preserved on one side in the fossil, is about 

 three-fourths of an inch wide and half an inch high. 



The tympanic bone, accompanying the former specimen, though much fractured, 



