40 MOSASAURUS. 
of the lower jaw, obtained by Dr. C. Thompson from the Marl of Freehold, Mon- 
mouth County, N. J. Restored 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 Historie de la Montagne de St. 
Pierre. The fragments compose twelve inches of the right dental bone, represented 
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 fangyand 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 crown 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, Monmouth 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 condyle, 
somewhat 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, 
