280 ENALIOSAURS. 



deep continuous furrow, and retained by slight ridges extending, 

 between the teeth, along the sides and bottom of the furrow (PI. 73, 

 fig. 9), and by the gum and the organized membranes continued into 

 the-groove and upon the base of the teeth. 



The germs of the new teeth are developed at the inner side of 

 the base of the old ones. Mr. Conybeare has given a figure of a 

 transverse section across the jaw-bone, (reproduced at PI. 73, fig. 7), 

 in which the new tooth (c) has penetrated the osseous substance of 

 the base of the old tooth (&), and its point has nearly entered the 

 remains of the pulp-cavity which has continued open in the crown of 

 the tooth (a). 



From the circumstance of the consolidation of the base of the 

 teeth in the Ichthyosaur Mr. Conybeare infers that they were 

 retained longer in the jaw than are the hollow teeth of the croco- 

 diles ; but the analogy of other Saurians, and the observation of two 

 new teeth at successive stages of formation at the base of an old 

 tooth, prove that the succession of new sets of teeth was repeated 

 more than once, though probably not so frequently as in the 

 crocodile. 



121. Plesiosaunis. — ^The teeth of the Plesiosaur are conical, long, 

 slender, curved and sharp-pointed ; they appear to retain their in- 

 ternal cavity, as in the teeth of a crocodile ; they have a very long 

 round fang or implanted base, which, in old teeth, contracts, as it 

 sinks into the jaw, and terminates almost in a point. 



The chief distinction, which the dental system offers between the 

 present and the preceding genus of Enaliosaur, is the loose implanta- 

 tion of the teeth of the Plesiosaur in separate alveoli. In thus 

 deviating from the Ichthyosaur, the Plesiosaur proportionally approx- 

 imates to the crocodilian type, and this affinity is likewise manifested 

 in the unequal size of the teeth, and the development of some of the 

 anterior ones into large tusks. 



The teeth are composed, as in the Ichthyosaur, of a body of 

 hard and simple dentine, covered at the crown by a coat of enamel, 

 and, at the base, by a coat of cement ; but the latter is relatively 

 thinner than in the Ichthyosaur, and is not inflected into the substance 

 of the dentine. The crown is characterized by well-defined narrow 



