WEALDEN DINOSAURS. 305 



which the crown is much worn down by the action of the opposite teeth below, and 

 the fang much absorbed by the stimulus of the growth of a succeeding tooth, a, the 

 primitive ridge ; b, b, the accessory ridges ; c, the angle of the anterior border. 

 Fig. 2 a, the inner side of the same tooth, showing the cavity produced by 

 absorption at the base d. Fig. 2 b, the grinding surface of the crown ; b, c, the two 

 facets produced by the attrition of two teeth of the lower jaw. 



Fig. 3. The inner side of a successional tooth of the right ramus of the lower jaw, 

 in which the crown is fully formed, with the beginning of the fang, but has not come 

 into use, and shows the lamello-serrated margin entire ; a, the primary ridge ; b, b, the 

 secondary ridges ; c, the anterior border. 



Fig. 3 a. The outer side of the same tooth, showing the widely open pulp- 

 cavity, p. 



Fig. 4, shows the outer side of a tooth from the left ramus of the lower jaw, the 

 crown of which had recently come into use ; c, the anterior margin of the base of the 

 crown ; d, the contracted end of the fang. Fig. 4 «, is an oblique view of the enamelled 

 inner side and posterior angular border, c, of the same tooth. 



Fig. 5, the outer side, fig. 5 a, the inner side, of a tooth from the left ramus of the 

 lower jaw, in which all the serrated part of the crown has been worn down in 

 mastication, and a great part of the fang renewed by absorption : the grinding surface 

 shows the two facets 6 and c, produced by the action of the two opposing teeth of the 

 upper jaw, and also the inequality due to the more rapid yielding of the softer 

 unenamcUed vaso-dentine, forming the outer half of the crown. 



Fig. 6. The inner side of the germ of a tooth of the lower jaw of a young Iguanodon, 

 probably from near the anterior end of the series. This tooth shows only the primary 

 ridge, and the entire serrated margin of the crown. 



Fig. 7. The outer side of a fully- formed and slightly worn tooth, from the upper 

 jaw of a young Iguanodon. 



Fig. 8. A magnified view of a longitudinal section of the upper part of the crown 

 of a slightly worn tooth of an Iguanodon, illustrative of the microscopic structure 

 described, pp. 270, 271, of the Section II on ' Cretaceous Reptiles.' e is the thin 

 layer of enamel which coats the outer side of the crown of the upper teeth and the 

 inner side of that of the lower teeth ; d is the hard or unvascular dentine, forming the 

 corresponding half of the crown ; v is the softer vaso-dentine, forming the inner half 

 of the upper and the outer half of the lower teeth ; m, the medullary or vascular canals ; 

 c, cement. 



Fig. 9, is a transverse section of the outer part of an upper tooth, more highly 

 magnified, of an Iguanodon, d the dentine, e the enamel. Fig. 9 a, a similar view of a 

 transverse section from the inner half of the crown of the same tooth; »«, the orifices of 

 the medullary canals. 



