CROCODILIANS. 



285 



The teeth of the Pleiosaur present varieties of form as well as of 

 size ; the rounding off of the angle between the ridged surfaces has 

 been already alluded to ; the smooth outer surface is sometimes so 

 convex, that the transverse section of the tooth is more eUiptical than 

 triangular. All the teeth of the Pleiosaur are slightly bent inwards 

 and backwards, but the smaller posterior teeth are most recurved, 

 and have the sharpest apex ; in the crown of these teeth, also, the 

 ordinary rounded or elliptical form of the cone is most nearly attained ; 

 but the distinction of the smooth external surface, and the ridged 

 internal surfaces of the crown of the tooth is retained, and would 

 suffice to characterize any of these teeth if found detached. 



The teeth of the Pleiosaur consist, like those of the Plesiosaur 

 and Crocodile, of a central body of compact dentine, with a coronal 

 investment of enamel, and a general covering of cement, of extreme 

 tenuity upon the crown, but thicker upon the base of the tooth. 



The dentine consists of fine calcigerous tubes, without admixture 

 of medullary canals ; the arrangement, division, secondary undula- 

 tions and branches of the calcigerous tubes correspond so closely with 

 those of the teeth of the Plesiosaur, as to render a particular descrip- 

 tion of them unnecessary. 



The germs of the successional teeth are developed at the inner 

 side of the bases of the old teeth , but do not penetrate these teeth ; 

 the apices of the new teeth make their appearance through foramina 

 situated at the inner side, and generally at the interspace of the 

 sockets of the old teeth. Here, therefore, as perhaps also in the 

 Pterodactyle, the growing teeth may be included in closed recesses of 

 the osseous substance of the jaw and emerge through tracts distinct 

 from the sockets of their predecessors ; but this is an exceptional 

 condition of the reproduction of the teeth in Reptiles. 



CROCODILIANS. 



123. The ancient writers on Natural History appear to have been 

 much struck with the great number of teeth in the crocodile : and 

 their descriptions were exaggerated to the tone of the impressions 

 thus produced. According to Achilles Tatius the crocodile had as 

 many teeth as there were days in the year : Alkazuin assigns it two 



