MOSASAURUS. . 47 



divide the outer surface of the crown into five, and the inner surface into seven 

 facets." 



The plaster cast, above mentioned, of the jav^s of the Maestricht Monitor, shovi^s 

 that the front teeth are narrower than those behind. The external surface of the 

 crown is comparatively narrow and slightly convex, while the internal surface is of 

 considerable extent, forming in section the transverse half of an ellipsoid. In 

 passing backward in the dental series it appears that the external surface of the 

 crowns increases in breadth and becomes more convex, while the internal surface 

 in a corresponding manner decreases. In the back teeth the crown appears to be 

 laterally compressed conical, with the external surface nearly as wide and convex 

 as the internal, so that a transverse section presents an ellipse with acute poles. 

 Thus the front teeth of the cast correspond Avith those usually described as charac- 

 -teristic of Mosasaurus, while the back ones are more like those supposed by Prof. 

 Owen to indicate a distinct genus, which he has named Leiodon. 



In most of the teeth, usually assigned to Mosasaurus, the inner and outer surfaces 

 of the crown are more or less distinctly subdivided into a series of narrow planes, 

 which are most evident towards the base of the crown. These planes are variable 

 in number, and are often slightly depressed or feebly concave. They are sometimes 

 multiplied towards the base, but become indistinct or even disappear at the summit. 

 In the plaster cast the divisional planes of the surfaces of the crown, while suffi- 

 ciently evident in the more anterior teeth, appear to be obsolete in those most pos- 

 terior, though it is true that their absence in the latter may arise from defective 

 modelling. 



In the Maestricht Monitor, Cuvier^ observes that, " the teeth are hollow only 

 during their development, as they are then in all other animals. They become 

 filled throughout their length, and are most frequently found entirely solid. They 

 complete their development in becoming attached to the jaw by means of an osseous 

 body very different in structure from that of the tooth, with which it is neverthe- 

 less intimately associated. The successional tooth originates in a special alveolus 

 produced at the same time, and it penetrates the osseous body of the tooth in use. 

 In enlarging the successional tooth finally detaches the osseous body from the jaw 

 with which it was organically united ; the body by a sort of necrosis being shed and 

 carrying with it the tooth it supported. Gradually the successional tooth, with its 

 body, improperly called its osseous root, assumes the position from which the old 

 one was removed." 



Subsequently, Cuvier,^ after remarking that " he had formerly committed the 

 error of calling the osseous structure, connecting the tooth with the jaw, the root," 

 observes that " he had since recognized it to be the dental pulp, which, instead of 

 remaining soft as in mammals, becomes ossified and identified with the alveolus." 

 Cuvier continues, " the tooth has no true root, but adheres strongly to the pulp 

 which secreted it, and is further held in connection with it by the remains of the 

 capsule which furnished the enamel, and which, by becoming ossified also, and 



» Ossemens Fossiles, Ed. 4, T. 10, p. 134. " Ossemens Fossiles, 136. 



