29G PALAEONTOLOGY. 



the succession al tooth prepared to supply the place of the 

 worn-out grinder. At the earlier stages of abrasion a sharp 

 edge is maintained at the ridged part of the tooth by means 

 of the enamel which covers that surface of the crown ; the 

 prominent ridges upon that surface give a sinuous contour to 

 the middle of the cutting edge, whilst its sides are jagged by 

 the lateral serrations. 



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

 sents a broad and nearly horizontal grinding surface (fig. 102), 

 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 grinding 

 surface ; the tooth in this stage has exchanged 

 the functions of an incisor for that of a molar, 

 and is prepared to give the final compression, or 

 comminution, to the coarsely divided vegetable 

 matters. The marginal edge of the incisive con- 

 Fig. 102. dition of the tooth and the median ridge of the 

 A worn tooth of molar stage are more effectually established by 

 guano on. ^ e introduction of a modification into the texture 

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

 isting Iguanse 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 form- 

 ing the corresponding walls of the tooth. This modification 

 must contribute in producing that inequality of texture and 

 of density which the broad and thick tooth of the Iguanodon 



