74 . FOSSIL REPTILIA OF THE 



the breach so excavated ; and as both this and the preceding tooth (fig. 8) are from the 

 lower jaw, the direction in which the fang is bent demonstrates that the germs of the 

 new teeth were developed from the inner angle of the bottom of the alveolar groove, 

 and affected the inner side of the base of the fully formed teeth, as in the Crocodiles. 

 In fig. 10 the crown and the smooth beginning of the fang of the successional tooth 

 have been completed, and it is seen enclosed by part of the debris of the old tooth 

 which it is about to replace. As in all young teeth, the crown is a thin shell of the 

 first-formed layers of dentine, with a thin coat of enamel, the ridges of which seem 

 not to have been quite completed. 



The teeth of the Ichthyosaurus are smaller at the two extremes than at the middle 

 of the series in both jaws ; and some modifications of form are presented in these 

 teeth, which do not, however, overpass the recognisable limits of the specific characters. 



Fig. 13 is a tooth probably from the back part of the series in the upper jaw, in 

 which the crown is less broad at its base and relatively longer than in the large 

 teeth from the middle of the series ; the rough expanded fang presents in transverse 

 section a long ellipse, with its angles truncated, making but a slight approach to the 

 quadrate figure of the fang of most of the larger teeth ; but in the fine striation of the 

 conical enamelled crown, in the smooth tract of unenamelled fang which precedes the 

 roughly striated expanded portion, and in the degree of expansion of this part, all the 

 distinctive characters of the Ichthyosaurus campylodon are preserved. 



Figure 14 is an incompletely formed tooth from the opposite end of the dental 

 series, in which the enamelled crown is unusually short and thick ; but the smooth 

 surface of the portion of the fang which has been formed, which continues to expand 

 to the widely open pulp-cavity, gives the character of the same species as fig. 13. 



In fig. 15, from the Upper Green-sand, we have a tooth in a more advanced stage 

 of formation : the roughened thickened part of the base has begun to be added ; but 

 this is still widely open, as is shown in fig. 1 5'. 



In Figure 1 6, a greater proportion of the rough expanded fang is completed, and 

 the basal outlet of the pulp-cavity is beginning to be encroached upon : but in these 

 young teeth the cement has not increased in such quantity as to be moulded into the 

 square form that is so characteristic of the old teeth. 



Jaws of the Ichthyosaurus campylodon, from the Cambridge Chalk. 



Tables XXV and XXVI. 



The portions of the upper and lower jaws discovered w 7 ith the teeth above 

 described, and containing several teeth of the same character in situ, are, as their 

 possessor, Mr. Carter, truly describes, the most characteristic relic of the Ichthyo- 

 saurian genus hitherto obtained from the Cretaceous Formations. 



The portion of the upper jaw includes an extent of tw T o feet of the premaxillary 



