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 intothe«groove and upon the base of the teeth.

The germs of the new teeth are developed at the inner side ofthe base of the old ones. Mr. Conybeare has given a figure of atransverse section across the jaw-bone, (reproduced at PI. 73, fig. 7),in which the new tooth (c) has penetrated the osseous substance ofthe base of the old tooth (6), and its point has nearly entered theremains of the pulp-cavity which has continued open in the crown ofthe tooth (a).

From the circumstance of the consolidation of the base of theteeth in the Ichthyosaur Mr. Conybeare infers that they wereretained longer in the jaw than are the hollow teeth of the croco-diles ; but the analogy of other Saurians, and the observation of twonew teeth at successive stages of formation at the base of an oldtooth, prove that the succession of new sets of teeth was repeatedmore than once, though probably not so frequently as in thecrocodile.

121. Plesiosaurus .—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 longround fang or implanted base, which, in old teeth, contracts, as itsinks into the jaw, and terminates almost in a point.

The chief distinction, which the dental system offers between thepresent and the preceding genus of Enaliosaur, is the loose implanta-tion of the teeth of the Plesiosaur in separate alveoli. In thusdeviating from the Ichthyosaur, the Plesiosaur proportionally approx-imates to the crocodilian type, and this affinity is likewise manifestedin the unequal size of the teeth, and the development of some of theanterior ones into large tusks.

The teeth are composed, as in the Ichthyosaur, of a body ofhard 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 relativelythinner than in the Ichthyosaur, and is not inflected into the substanceof the dentine. The crown is characterized by well-defined narrow