66 MOSASAURUS. 
long, and is divided by minutely denticulated ridges into two smooth surfaces, of 
which the 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 lines 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 cf 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 worn, 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 (4) 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 Mosasaurus 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 number of pterygoid teeth in the 
Maestricht Monitor. The anterior teeth, however, are very much larger than in 
* The artist neglected to represent in the figure the bottom of the fang, visible through the vasculo- 
neural foramen, so that the tooth looks actually acrodont. 
