46 MOSASAURUS. 
and a quarter in transverse diameter, and a little less in the opposite diameter. 
The lower end of the bone expands transversely, and ends in a long, curved ellip- 
soid surface, apparently subdivided to articulate with three carpal bones. The 
length of this surface transversely is two inches and a quarter, its breadth three- 
quarters of an inch. 
Specimens of isolated teeth, possessing the general characters assigned to those 
of the Maestricht Monitor or Mosasaurus, but exhibiting considerable diversity of 
size and form, are frequently discovered in the deposits of the Cretaceous period 
of the United States. A number of such teeth are contained in the cabinet of the 
Academy of Natural Sciences, and others have been loaned to me for examination, 
but I find it difficult to decide whether they belong to one or more species of the 
genus or to several distinct genera. In attempting to determine the limit of varia- 
tion in the form of the teeth of Mosasawrus, I have greatly felt the want of a careful 
description in detail, accompanied by accurate figures, of the fine specimen of the 
jaws and teeth, upon which the genus was founded, and which is now preserved in 
the museum of the Jardin des Plantes, at Paris. I have had access to an excellent 
plaster cast of the specimen presented by the Directors of the latter institution to 
the American Philosophical Society, but in many respects it fails to show nice 
shades of character, which are, no doubt, to be observed in the original, or wnich 
might be the subject of description and accurate delineation. The descriptions and 
figures by Faujas-Saint-Fond, Cuvier, and others, though sufficiently characteristic 
of the genus, and mainly correct, are not given with the detail and precision that 
are required for comparison of the specimens I have the opportunity of examining. 
‘uvier’ briefly describes the teeth of Mosasawrus in general. He observes, that 
they are all pyramidal and slightly curved; their external face is plane, and defined 
by two acute ridges from the internal face, which is round, or rather demi-conical. 
Subsequently? he remarks, that the ridges are entire and without denticulations. 
He does not refer to the existence of divisional planes upon the teeth; and an 
inspection of undoubted teeth of the genus leaves no question that the ridges of 
the crown are minutely denticulate in the unworn condition. An examination of 
the cast, above mentioned, also proves that there is some diversity in the shape of 
the teeth in different parts of the series. 
 Goldfuss,’ in describing the teeth of Mosasaurus, says they are slightly compressed 
laterally, towards the apex feebly curved backwards, and are divided into a larger 
inner and smaller outer half by an acute linear ridge, which is transversely striated. 
Their surface appears polygonal; the outer surface presenting five, the inner seven 
pyramidal planes. Prof. Owen,' in referring to the description of Goldfuss, remarks, 
“the feeble indications of angles observable in some of the teeth, those of the upper 
jaw chiefly, of the Mosasawrus Hoffmanni, do not bear out the term ‘ polygonal,’ 
which he applies to the crowns of that species as well as to those of his Mosasaurus 
Mazximiliani ; still less can I find these angles so constant and regular as to 
1 Ossemens Fossiles, Ed. 4, T. X, p. 144. a Ibid., p, 145. 
* Schidelbau des Mosasaurus; Nov. Act. Acad. C. L. C. Nat. Cur., Vol. 21, p. 178. 
+ British Fossil Reptiles, p. 185. 
