184 STRUCTURE. 



is the intermingling of cylindrical processes of the pulp cavity, in the 

 form of medullary canals, with the finer tubular structure. (1) Another 

 modification is that in which the dentine maintains its normal 

 structure, but is folded inwardly upon itself, so as to produce a deep 

 longitudinal indentation on one side of the tooth : it is the expansion 

 of the bottom of such a longitudinal deep fold that forms the central 

 canal of the venom-fang of the serpent ; but a glance at PI. 65 a, 

 will show that, notwithstanding the singularly modified disposition 

 of the dentine (&), its structure remains unaltered : and although the 

 pulp-cavity {a) is reduced to the form of a crescentic fissure, the calci- 

 gerous tubes continue to radiate from it, according to the usual law. 

 By a similar inflection of many vertical longitudinal folds of the external 

 cement and external surface of the tooth, at regular intervals, around 

 the entire circumference of the tooth, and by a corresponding extension 

 of radiated processes of the pulp-cavity and dentine into the interspaces 

 of such inflected and converging folds, a modification of dental struc- 

 ture is estabhshed in certain extinct reptiles, which, by the various 

 sinuosities of the interblended folds of cement and processes of 

 dentine, with the partial dilatations of the radiated pulp-cavity, 

 produces the most compUcated structure that has yet been met with 

 in the teeth of any animal, (PI. 64 a) . But this complication is never- 

 theless referable to a modification of form or arrangement rather 

 than of structure of the dental tissues : the calcigerous tubes in each 

 sinuous lobe of dentine, in the most complex tooth of the Labyrin- 

 thodon, exhibit the same general disposition and course as in the 

 fang of the Serpent and in the still more simple tooth of the Saurian. 



In the Iguanodon (PI. 71) the fine-tubed dentine is traversed 

 by medullary canals which run at pretty definite intervals through 

 the dentine, parallel with the calcigerous tubes, as in that coarser 

 kind of dentine which characterizes the teeth of the sloth and mega- 

 therium, and which, in connection with the complex form of the 

 teeth of the Iguanodon, peculiarly adapted that gigantic reptile for a 

 vegetable diet. 



The cement is simply and minutely cellular upon the crown 

 of the tooth, but it exhibits the radiated cells at the base of the tooth in 

 the anourous Batrachians, and Saurians. The enamel is subtransparent, 



Cl) Transactions of the British Association, 1838, p. 144. 



