FOSSIL REPTILES. 351 



The teeth of all the ichthyosauri are conical, and their crown 

 is enamelled, and striated longitudinally as in the crocodiles. 

 It is more or less sharp, more or less swelled, more or less 

 compressed, according to the species. The root is more 

 bulky, not enamelled, but it is striated like the crown. 



These teeth remain a long time hollow internally. They 

 are not enchased in alveoli as deep and close as those of the 

 crocodile, nor are they naked on the internal side like those of 

 the lizards. But it appears that they are simply ranged in a 

 deep furrow of the maxillary bone, the bottom of which alone 

 is hollowed with fosses corresponding to each tooth. 



Their mode of replacement is analogous to that of the cro- 

 codile, with this difference, that in the crocodile whose teeth 

 are always hollow, the new tooth penetrates into the interior of 

 the old ; while in the ichthyosaurus, the root being ossified, the 

 new tooth penetrates only into the cavity formed by the caries, 

 a cavity which augments in proportion as the new tooth in- 

 creases in size, and which, finally, causing the root to disap- 

 pear, determines the fall of the crown of the old tooth. 



This crown of the tooth preserves still in its anterior a 

 cavity, usually filled with spath, long after the root is ossified. 

 The new root begins to ossify even before the old tooth has 

 fallen . 



The number of teeth is considerable. Mr. Conybeare does 

 not reckon less than thirty on each side in each jaw. Sir 

 Everard Home has pointed out forty-five on each side in 

 each jaw, in the individual figured in the ^' Philosophical 

 Transactions of 1820." 



Messrs. Conybeare and De Labeche have found sufficient 

 differences between these teeth, to deduce from them the cha- 

 racters of four distinct species. 



The Ichthyosaurus communis, whose teeth have a conical 

 crown, moderately sharp, slightly hooked, and deeply striated. 

 This species is, in general, large, and to it the most gigantic 

 individuals belonged. 



