268 PALAEONTOLOGY 



edge, whilst its sides are jagged by the lateral serrations. The 

 adaptation of this admirable dental instrument to the cropping 

 and comminution of such tough vegetable food as the Clathrariae 

 and similar plants, which are found buried with the Iguanodon, 

 is pointed out by Dr. Buckland, with his usual felicity of 

 illustration, in his Bridgewater Treatise, vol. i., p. 240. 



When the crown is worn away beyond the enamel, it pre- 

 sents a broad and nearly horizontal grinding surface (fig. 79), 

 and now another dental substance is brought 

 into use, to give an inequality to that surface : 

 this is the ossified remnant of the pulp, which, 

 being firmer than the surrounding dentine, forms 

 a slight transverse ridge in the middle of the 

 Fiii . 7 , ( grinding surface ; the tooth in this stage has 

 A worn tooth of exchanged the functions of an incisor for that 

 the Iguanodon. Q £ a mo p ar? anc i jg prepared to give I he final 

 (•(impression, or comminution, to the coarsely divided vege- 

 table matters. 



The marginal edge of the incisive condition of the tooth 

 and the median ridge of the molar stage are more effectually 

 established by the introduction of a modification into the 

 texture of the dentine, by which it is rendered softer than in 

 the existing Iguana? and other reptiles, and more easily worn 

 away. This is effected by an arrest of the calcifying process 

 along certain cylindrical tracts of the pulp, which is thus con- 

 tinued, in the form of medullary canals, analogous to those in 

 the soft dentine of the Megatherium's grinder, from the central 

 cavity, at pretty regular intervals, parallel with the dentinal 

 tubes, nearly to the surface of the tooth. The medullary canals 

 radiate from the internal (upper jaw) or external (lower jaw) 

 sides of the pulp-cavity, and are confined to the dentine forming 

 the corresponding walls of the tooth. Their diameter is p^ )0 th 

 of an inch. They are separated by pretty regular intervals, 

 equal to from six to eight of their own diameters. They 



