MOSASAURUS. 51 
enamel border of the crown the dentine is defined from the cementum of the 
extra-alveolar portion of the fang by a more amorphous bond of union of the two 
structures, as indicated by the clear dividing line in the figures above mentioned. 
The fang (Fig. 3, 6, Plate XX) is composed of cementum or bone, as represented 
in Fig. 4, 5, 6, 6, of the same plate. It is mainly composed of vertical osseous 
fibres, pervaded by numerous vascular canals pursuing the same course as the 
former. It is of much finer texture than the bone of the jaw with which it may 
be intimately coossified, and is admirably adapted to sustain the crowns of the teeth, 
both as regards its organic and its physical functions.! 
The teeth of Mosasaurus, belonging to the functional series or those in use, were 
succeeded by a new set which underwent their development at the postero-internal 
portion of the alveoli occupied by the former. For the reception of the growing 
crowns of the new teeth the fangs of the functional series were gradually excavated 
through absorption of their structure, in a direction from within obliquely outward 
and forward, upward and downward. At first the inner parapet of the jaw slightly 
contributed to the parietes of the cavity for the new tooth, but, with this trifling 
exception, it was through excavation of the contiguous fang of the functional tooth 
that the former was accommodated. In the progress of the excavation, the pulp 
cavity of the functional tooth became exposed and then cut off from communica- 
tion with the nerves and bloodvessels which supplied its contained pulp. The 
fossil specimens further indicate that it was during the progress of the excavation 
of the fangs of the functional teeth that these became coossified with their alveoli, 
as if to resist a tendency to expulsion from the jaw. 
The cavities for the new teeth, in the fossils, are ovoid in form, and open at the 
postero-internal part of the extra-alveolar portion of the base of the fangs of the 
functional teeth; and from the opening the apex of the new tooth is seen pro- 
truding. 
After the development of the crown of the new tooth the fang was produced, 
and the increase gradually became so great at the next step as to have converted 
the fang of the old tooth into a large capsule, surmounted by its crown still in use. 
With the advance of growth of the new tooth the crown of the old one became so 
enfeebled in its connection with its excavated fang as readily to be broken off by 
external violence, or to be displaced by the continued growth of the fang and pro- 
trusion of the crown of the new tooth. ‘The fang of the latter continued its growth 
within a mere cylinder of the fang of the old tooth until its crown was made to 
assume a position in the functional series. 
The development of the new tooth was scarcely completed before a successor 
commenced the same process, and thus one tooth was followed by another throughout 
the life of the animal, as in recent Reptiles (Plate IX, Figs. 1, 4, 5,6; Plate X, 
Figs. 1, 4, 7, 8, 10; Plate XI, Figs. 4, 5, 6, 8, 10). 
* Owen says, ‘‘ The expanded base of the tooth,” referring to what has been mentioned above as 
the extra-alveolar portion of the fang, “is composed of a mere irregular mass of dentine, which, by 
its progressive subdivisions into vertical columnar processes, assumes a structure resembling that of 
true bone.’ Odontography, 259. 
