CRETACEOUS ICHTHYOSAURS. 227 



been only checked by the resistance of the alveolar walls on the outer and inner sides 

 of the tooth, and by the contiguous teeth before and behind. Thus, by this thickening 

 of the fang, the teeth must have become wedged together in the common alveolar 

 groove, and the absence of partitions completing the sockets must have been in some 

 measure compensated by this firm impaction. This is shown in the part of the fractured 

 jaws. (PI. 3, fig. 2.) 



Figs. 3 and 4, PI. 1, give two views of a portion of the alveolar groove with 

 one tooth thus squarely wedged in its place, part of the adjoining tooth on one side, 

 and part of the socket on the other, in which a thin bony partition had been formed 

 for a short extent of the base of the tooth. The extent of the square root in the 

 direction of the long axis of the jaw, fig. 4, is commonly greater than the transverse 

 diameter of the same root, fig. 3. The tooth is never wholly consolidated even in this 

 fully developed state of the fang : a remnant of the uncalcified pulp has always been 

 retained in the central dentinal part of the enlarged fang, after the crown has been 

 completed. This is shown in the fractured specimen, fig. 5, in which the square fang 

 beyond the cavity, o, is one solid mass of coarse cement ; and more clearly in the 

 transverse section, fig. 6, in which c is the cement, (/ the dentine, and a the pulp-cavity. 

 The view given at fig. 6' shows the consolidated base of the thickened fang, — a 

 character by which the teeth of the Ichthyosaurs diifer from those of almost all other 

 Saurians, and especially from the Crocodiles, in which the base of the tooth always 

 remains widely open. 



Notwithstanding, however, the resistance which must be opposed by the thickened 

 and consolidated root of the tooth of the Ichthyosaur, it is affected by the germ of the 

 succeeding tooth in the same way as in the Crocodilia. I have seldom, indeed, seen 

 the process better illustrated than in a series of the teeth of the IcJdhyosaunis cam- 

 pylodon in Mr. Carter's collection, obtained from the Chalk in the neighbourhood of 

 Cambridge. 



Fig. 7, PI. 1, is a tooth with a thick, straight, square fang ; probably, therefore, from 

 the upper jaw, which shows on one of the broader sides of the base a shallow elliptical 

 depression, o. This is caused by the progressive absorption excited by the pressure 

 of the soft matrLx of the successional tooth which was in the course of development at 

 the angle of the alveolar groove on the inner side of the base of the tooth in place. 

 In the Ichthyosaur's tooth the absorption causes a simple excavation in the substance 

 of the thick cement ; but in the Crocodile's the same process speedily penetrates the 

 thinner wall of the large cavity in the base of the tooth, as is shown in the figure of 

 that of an Alligator (fig. 11), where a circular aperture is the result of the pressure. As 

 the new tooth of the Ichthyosaur grows, the thick cement of the old tooth yields, 

 and the reduced pulp-cavity in the centre of the fang is penetrated, as is shown at 

 fig. 8, o, where the absorbent process has extended nearly across the whole solid base of 

 the fang, fig. 8'. In fig. 9 the germ of the tooth is preserved, which has penetrated 



k 



