ENALIOSAURS.

279

of a fibrous structure, the lines being vertical to the surface of thetooth.

The coronal cement appears only as a line of substance moreopaque than the enamel which it invests; it augments in thick-ness at the base of the tooth, where the radiated corpusclesor cells that characterize its structure are very conspicuous; thecement is inflected at each of the basal grooves in the form of ashort, straight and simple vertical fold into the substance of thedentine. The peripheral portion of the basal dentine is thus divided,to the extent represented in PI. 64 b, fig. 3, into a correspondingnumber of processes; fissures of the pulp-cavity radiate to theirbases, becoming there the centres of divergence of as many series ofcalcigerous tubes, which obey in their course the usual law of verti-cality to the external surface of the dentine. This structure can be seenonly in a transverse section of the base of the tooth: its correspon-dence with that of the apex of the crown of the teeth of the Laby-rinthodon will be obvious on comparing fig. 3, PI. 64 b, with fig. 1,PI. 63, b, and, as has been already stated, it gave the key to thenature and principle of the complicated labyrinthic interblending ofdentine and cement, which was first observed in the great tusk ofthe Labyrinthodon Jaegeri.

The remains of the pulp, after the formation of the due quantityof dentine, became converted, as in the pleodont lizards, by a pro-cess of coarse ossification into a reticulate fibrous or spongy bone(l) ;but it continues open at the crown after the basal part of the tooth isthus consolidated, as shown in the longitudinal section (PI. 73, fig. 8),wherein a is the pulp-cavity filled with crystallized spath, b theossified pulp at the base of the tooth. The radiated cells or corpusclesare very conspicuous in both this bone and the external cement.

The chief peculiarity of the dental system of the Ichthyosauris the mode of implantation of the teeth ; instead of being anchylosedto the bottom and side of a continuous shallow groove, as in mostLacertians, or implanted in distinct sockets, as in the Thecodon,JVlegalosaur or Pterodactyle, they are lodged loosely in a long and

(1) “ The tooth in these genera becomes completely solid, its interior cavity being filled upby the ossification of the pulpy substance.”—Conybeare, loc. cit. p. 106.