46 MOSASAURUS. 



and a quarter in transverse diameter, and a little less in the opposite diameter. 

 The lower end of tlie 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 Mosasaurus, 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 which 

 might be the subject of description and accurate delineation. The descriptions and 

 figures by Fauj as-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. 



Cuvier^ briefly describes the teeth of Mosasaurus 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 difi'erent 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 Mosasaurus 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 

 Maximiliani ; still less can I find these angles so constant and regular as to 



' Ossemens Fossiles, Ed. 4, T. X, p. 144. » Ibid., p. 145. 



" Schadelbau des Mosasaurus; Nov. Act. Acad. C. L. C. Nat. Cur., Vol. 21, p. HS. 

 * British Fossil Reptiles, p. 185. 



