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, b, Plate XX) is composed of cementum or bone, as represented 

 in Fig. 4, 5, 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 fmictional 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 communicar 

 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 ofi" 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 Eeptiles (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-alveola.- 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. 



