(;(; MOSASAFRUS. 



lonsj^, and is divided by minutely denticulated ridges into two smooth surfaces, of 

 which tlic outer is slightly the larger. The transverse section, as represented in 

 the accompanying outline, No. 24, is elliptical, and the antero-posterior diameter at 

 the slightly constricted base is nine Imcs and three-quarters ; transversely six lines 

 and a half 



The extra-alveolar portion of the fang is half an inch high ; sixteen lines in 

 diameter antero-posteriorly at the alveolar border, and eleven lines transversely. 

 The intra-alveolar portion of the fang is an inch long, and encroaches for half its 

 length within the dental canal.' Postero-internally, together with the contiguous 

 portion of the jaw, it is excavated into a cavity which contains the crown of a 

 successional tooth. 



The alveolus in advance retains the outer half of a coossified fang, which was 

 about a third excavated for a successor. The portions of the alveoli at the anterior 

 and posterior border have the appearance as if their former occupants had been 

 lost entire, crown and fang together. 



A fragment of the left dental bone, of which Fig. 4, Plate XI, represents an 

 inner view of part of the specimen, nearly corresponds with the former one rf the 

 opposite side. The entire tooth it contains corresponds in position with that in 

 advance of the one preserved in the former. The tooth larger than in the preceding 

 specimen is like it in form. The crown, with its apex considerably ivorn, thus 

 reduced, is thirteen lines long ; is nine lines and three-quarters in diameter at base 

 antero-posteriorly, and seven lines and a half transversely. The fang is two inches 

 long, and the dental canal pursues its course just external to its bottom. 



The specimen is especially interesting from the circumstance that the successional 

 tooth (c), inclosed in the cavity of the fang (b) in advance, having been accidentally 

 partially broken away, exhibits in the interior a minute successor {d). It thus 

 appears that in the succession of development of the teeth of Mosasaums a new 

 tooth originates within its predecessor, while this is still contained in the excavated 

 fang of the tooth occupying a functional position at the border of the jaw. As the 

 latter is displaced by its successor it would appear that as the crown of this pro- 

 trudes from the jaw the new tooth is excluded from its place, and is made to 

 assume a position on the exterior of the fang of its parent. The new tooth, as 

 if desirous of once more obtaining admission into the position from which it had 

 been excluded, in its growth induces absorption of the fang of its predecessor so 

 as to accommodate its increasing size. 



Two fragments of the right pterygoid bone, represented in Figs. 1, 2, Plate XI. 

 The larger fragment contains a tooth and the fangs and alveoli of four others ; the 

 smaller fragment contains two teeth, part of another, and part of a large succes- 

 sional cavity which appears to correspond with a similar part at the end of the 

 larger fragment. It would thus appear that there were eight teeth to the full 

 series, corresponding in this respect with the nvimber of pterygoid teeth in the 

 Maestricht Monitor. The anterior teeth, however, are very much larger than in 



' The artist noglectei} to represent in the figure the bottom of the fang, visible through the vaseulo- 

 ueural foraiuon, so that the tooth looks actually acrodont. 



