MOSASAURUS. 73 
The crowns are conical, with a circular base, and are strongly curved backward. 
They are nearly equally divided by an acute ridge, externally and internally, which 
only extends about half the length of the crowns from their apex. The anterior 
and posterior surfaces are strongly and comparatively coarsely striated. 
The crown of the first tooth is two lines long, that of the second two lines and 
three-fourths, of the third four lines; the others are broken at the apex. The 
diameter of the first at base is one line and a half; of the last tooth three lines. 
The fangs of the anterior three teeth are coossified together, but are separated 
from that of the fourth tooth by a wide crescentoid fissure. Following the fourth 
tooth is a thimble-like socket half an inch deep at its outer wall, and five lines 
wide. The fangs of the second, fourth, and last teeth present excavations at their 
inner part posteriorly for the accommodation of successors. 
Among the many specimens of teeth which have been indicated and described, it 
may be noticed that there are a number of well-marked varieties of form which 
might be viewed as representing different genera and species of Mosasawroids, were 
it not that through intermediate forms they more or less graduate into one another. 
Referring to the plates IX, X, XI, in which nearly all the varieties of teeth have 
been figured, the gradation of form can readily be traced. If most of the specimens 
belong to the same species, the variation of form is certainly remarkable; but on 
the other hand, if the well-marked varieties of form be considered as indicating dis- 
tinct species, then the number of these is far greater than any one had suspected. 
a, The specimens described under Numbers 1, 3, 4, 5, 7, 8, 10, represented in 
Figs. 1, 2, 3, Plate IX; Figs. 1, 2, 3. Plate X, exhibit teeth answering to the 
usual description of authors as characteristic of the great Mosasaurus. The crown, 
long, conical, curved especially towards the apex, and unequally divided by a 
pair of acute ridges into two surfaces, of which the inner is the more extensive and 
convex, and both are subdivided into narrow planes. ‘The specimens present a 
wide range in size, and differ in the relative extent of their two surfaces, and in 
the number and distinctness of their subdivisional planes. 
b. Number 9 resembles those intermediate in size of the foregoing, but has the 
crown more compressed and less unequally divided by the acute ridges. 
ce. Number 6, Fig. 6, Plate X, resembles the larger specimens of a, but has the 
crown somewhat compressed, is less unequally divided by the acute ridges, has 
more numerous subdivisional planes but less distinct, and it is striated. 
d. The eight large specimens of Number 14, most of which are represented in 
Figs. 5, 6, Plate IX; Figs. 7-10, Plate X, resemble the larger ones of a, but the 
crowns are nearly or quite equally divided by the acute ridges. In several the 
crown is nearly as full as in a, but in others is compressed; and in one specimen 
Fig. 4, Plate X, there is but a single acute ridge to the crown. The ninth tooth of . 
the series Number 14, Fig. 5, Plate XI, supposed to have belonged to the same 
individual, is a miniature form of the specimen just indicated with a single acute 
ridge, except that it is destitute of subdivisional planes. 
e. Numbers 11, 12, 13, the smaller specimen of Number 18, and Numbers 19 and 
31, Figs. 4, 8, 10, Plate IX; Figs. 5, 11, Plate.X, and Figs. 9, 12, Plate XI, 
exhibit crowns of teeth having nearly the form of those of a, but totally devoid of 
10 April, 1865. 
