ENALIOSAURS. 279 



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

 tooth. 



The coronal cement appears only as a line of substance more 

 opaque than the enamel which it invests ; it augments in thick- 

 ness at the base of the tooth, where the radiated corpuscles 

 or cells that characterize its structure are very conspicuous ; the 

 cement is inflected at each of the basal grooves in the form of a 

 short, straight and simple vertical fold into the substance of the 

 dentine. The peripheral portion of the basal dentine is thus divided, 

 to the extent represented in PI. 64 b, fig. 3, into a corresponding 

 number of processes ; fissures of the pulp-cavity radiate to their 

 bases, becoming there the centres of divergence of as many series of 

 calcigerous tubes, which obey in their course the usual law of verti- 

 cality to the external surface of the dentine. This structure can be seen 

 only 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, PL 64 b, with fig. 1, 

 PI. 63, B, and, as has been already stated, it gave the key to the 

 nature and principle of the complicated labyrinthic interblending of 

 dentine and cement, which was first observed in the great tusk of 

 the Labyrinthodon Jaegeri. 



The remains of the pulp, after the formation of the due quantity 

 of 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 is 

 thus consolidated, as shown in the longitudinal section (PL 73, fig. 8), 

 wherein a is the pulp-cavity filled with crystaUized spath, b the 

 ossified pulp at the base of the tooth. The radiated cells or corpuscles 

 are very conspicuous in both this bone and the external cement. 



The chief peculiarity of the dental system of the Ichthyosaur 

 is the mode of implantation of the teeth ; instead of being anchylosed 

 to the bottom and side of a continuous shallow groove, as in most 

 Lacertians, or implanted in distinct sockets, as in the Thecodon, 

 Megalosaur or Pterodactyle, they are lodged loosely in a long and 



(1) " The tooth in these genera becomes completely solid, its interior cavity being filled up 

 by the ossification of the pulpy substance." — Conybeare, loc. tit. p. 106. 



