276 ENALIOSAURS. 



ed at the base by a thick layer of cement, and at the crown by a 

 layer of enamel, which is itself covered by a very thin coat of ce- 

 ment ; the pulp-cavity is more or less occupied in fully-formed teeth 

 by a coarse bone. 



The external surface of the tooth is marked by longitudinal im- 

 pressions and ridges, but the teeth vary both as to outward sculp- 

 turing and general form in the different species. (1) 



The teeth present the largest size in the Ichthyosauri platyodon 

 and communis, and they are smallest and most slender in the 

 Ich. tenuirostris . 



Ich. communis, (PI. 73, fig. 4). This species is characterized by 

 teeth which have a cylindrical, sometimes sub-ventricose base, and a 

 round, conical, slightly aduncate crown, tapering more rapidly and 

 less regularly to the apex than in the other species ; the apex is not 

 very acute. The crown is traversed by moderately fine and close -set 

 longitudinal furrows, and the base is sculptured by coarse and deep 

 grooves which separate longitudinal convex ridges. 



There are from forty to fifty teeth on each side of the upper jaw, 

 eighteen of which are implanted in the superior maxillary bone ; in 

 the lower jaw there are on each side between twenty-five and thirty 

 teeth. The fine coronal grooves are abruptly divided from the coarse 

 basal ones at the terminal line of the enamel. The basal ridges are 

 sometimes transversely scored in large teeth, and bifurcate and pro- 

 gressively diminish in breadth as they approach the contracted 

 extremity of the tooth ; the base of such teeth sometimes resembles a 

 small contracted pentacrinite. 



Ich. platyodon (PI. 73, figs. 3 & 6). — The teeth of this species 

 present a ventricose base, and a conical, sub-compressed, subincurved 

 crown, the outer and inner sides of which meet at two opposite sharp 

 edges, which terminate above at the acute apex of the crown, which is 

 thus adapted for both piercing and cutting. 



The basal longitudinal grooves and ridges of the tooth are as 



(1) PL 73, 73 A & 74. 



(2) The principal varieties of the form of the teeth, and the species thereby indicated were 

 first observed by M. Delabeche, and are described by Mr. Conybeare in his beautiful memoir 

 in the Geol. Trans, vol. vi, p. 103. 



