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 Mosasauroids, were 

 it not that through intermediate forms they more or less graduate into one another. 

 Eeferring 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 crovni, 

 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. 



c. 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 Avith 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, 1S65. 



