M OS A SALT KITS. 49 



municating through a canal with a fimnel-shaped pit at the end of the fang. See 

 Plates IX, X, XI. 



The conical crowns of the teeth are curved backward with an inclination inward; 

 the curvature being more rapid approaching the apex. They are generally divided 

 in front and behmd by an acute ridge into an inner and an outer surface. In some 

 teetli, apparently belonging to the most posterior of the dental series of the jaws, 

 and to those of the pterygoid bones, there is only one ridge, which is situated along 

 the back or concave border of the crown. The ridges exhibit a minutely crimped 

 and sub-denticulated arrangement,^ which was obliterated by wearing. 



The proportionate extent of the inner and outer surfaces of the crown, as defined 

 by the two ridges above indicated, varies very much in the different specimens of 

 fossil teeth, apparently accordmg to the position the latter occupied in the dental 

 series. 



In those teeth, which I suspect to belong to the anterior part of the dental series, 

 tlie crown has the form corresponding with the descriptions Avhich have usually been 

 given as characteristic in general of the teeth of Mosasaurus (Plate IX, Figs. 1, 

 2, 3; Plate X, Figs. 1, 2, 3). It is very unequally divided by the acute ridges; the 

 inner surface occupying two-thirds or more of the extent of the crown. The con- 

 vexity or transverse curvature of the outer surface forms a short segment of a com- 

 paratively large circle ; that of the inner surface forms one-half to two-thirds or 

 more of a circle, whose diameter is that of the crown. The transverse section of 

 the crown might be appropriately called shield-shaped, as represented in the wood- 

 cut outlines, Nos. 1, 2, 3. 



In those teeth which are supposed to belong to the middle of the dental series 

 the disproportion between the outer and inner surfaces of the crown is comparatively 

 trifling, and the transverse section is circular or nearly so (Plate IX, Figs. 5, 6 ; 

 Plate X, Figs. 7-9). 



Teeth attributed to the more posterior part of the dental series have their crowns 

 compressed from within outwardly and nearly equally divided by the acute ridges, 

 and in transverse section are elliptical with acute poles (Plate X, Fig. 10). 



Finally, the last teeth of the series have compressed crowns, with a single ridge, 

 and an ovate transverse section (Plate X, Fig. 4). 



The inner and outer surfaces of the crowns in most of the fossil teeth are une- 

 qually subdivided into narrow planes, variable in number. They slightly multiply 

 towards the base of the crown, and become fewer and less distinct, or altogether 

 disappear towards the apex. They vary in degree of distinctness in different teeth, 

 and in many do not exist at all (Plates IX, X, XI). 



he indicates as a maxillary and a palatal tooth of Mosasaurus Eoffmanni. The tooth represented in 

 his figure 4 looks as if it may have belonged to near the middle of the dental series. The inner side 

 of its fang exhibits a small lenticular excavation ; part of the receptacle of a successional tooth. 

 rig. 5, represented as a palatal or pterygoid tooth, I suspect rather to belong to the back part of 

 the maxillary series. The two figures are reproduced by Gibbes in Plate I, of his Memoir on the 

 Mosasaurus. 



' This arrangement appears not to have been noticed by Cuvier in the teeth of the Maestricht 

 Monitor. Ossemens Fossiles, T. X, 145. 



7 April, 1865. 



