72 FOSSIL REPTILIA OF THE 



of that part of the fang next the crown, in the Ichthyosaurus campijlodon, is due to a 

 thick coat of cement ; the dentine so covered shows a fluted character, only differing 

 from that of the teeth of the Ichthjosaurus communis in being more regular and some- 

 what finer. This is shown in T. XXIII, fig. 4. 



Not any of the detached teetli discovered with the above-described portions of jaw 

 present any well-marked curvature of the base. The characteristics of the teeth of 

 the Ichthyosaurus campylodon are best displayed in those specimens that have been 

 obtained from the cretaceous deposits in Cambridgeshire. 



Teeth of Ichthyosaurus campylodon. Tab. XXIV. 



The detached teeth from the Cambridge Chalk and Green-sand present two 

 modifications of form : the majority are straight, the rest curved, chiefly owing to a 

 slight inward bending of the thickened fang. These latter have been proved to come 

 from the lower jaw, and the curvature relates, as Mr. Carter has well remarked, to 

 the more oblique direction outwards of the alveolar groove in that jaw, which is 

 compensated by the curvature of the teeth, the crowns of which are thereby brought 

 into more direct apposition with the teeth above.* 



The enameled crown in all the teeth (figs. 1, 2, 6 c) is a cone, short and thick in the 

 largest teeth, with a circular or very full elliptical transverse section ; it is a longer and 

 narrower cone in most of the smaller teeth. The ridges of the enamel are numerous 

 and fine, not always of equal thickness ; the intervening grooves are rather narrower 

 than the ridges. In some teeth, shorter and narrower ridges are seen in the basal 

 intervals of the longer ridges : in other teeth the ridges are thicker at the base of the 

 crown, and are occasionally impressed or divided there by a shorter longitudinal 

 groove. All the ridges subside before they reach the apex of the crown, which is 

 smooth. The enamel terminates at the base of the crown by a thin well-defined 

 border. The tooth continues to expand beyond this border, and, for an extent 

 varying from one third to one fifth of the entire fang. The surface is smooth ; not 

 any of the longitudinal furrows or ridges of the enameled crown being continued upon 

 that part of the cement-covered fang. In a few teeth, the base of the crown is w-ell 

 defined, as Mr. Carter has remarked, by an annular projection, T. XXIV, figs. 3 and 4. 

 The rest of the base or fang of the tooth, beyond the smooth part, presents coarse 

 longitudinal ridges and grooves, is much expanded in most of the teeth, and in many 

 it presents, as in the tooth figured and described by Mr. Carter,! a square shape. 

 This character is best marked in the straight teeth from the upper jaw ; it arises out 

 of the progressive growth of the osseous cement of the fang, w^hich seems to have 



* London Geological Journal, vol. i, p. 9. 

 f Loc. cit., p. 8, figs, a and h. 



