310 PALAEONTOLOGY. 



Hcclidosaurus by the marginal serrations of the apical half of 

 the crown progressively increasing in size from the apex to 

 the base of that angular part of the tooth, the two basal points 

 resembling spines, and terminating respectively, or forming 

 the confluence of, the two thickened ridges bounding the 

 fore and hind borders of the basal half of the crown. From 

 Macellodon it differs in the swollen borders of the basal 

 half and the stronger serration of the apical half of the 

 crown. The similarly expanded crown of the tooth of Car- 

 diodon* has thicker, and apparently not serrate margins, it is 

 not divided into a basal and apical portion, and the apex 

 is more obtuse. In Hylceosawus the crown is thicker and 

 less expanded than in Echinodus; the borders of the apical 

 half are usually abraded by masticatory acts, shew no marks 

 of serration, and meet at an angle of 80° ; but the crowns of 

 the teeth were in contact, as in Echinodon. The more com- 

 plex structure of the teeth of Iguanodon appears, nevertheless, 

 to be due to additions superposed upon a type of tooth which 

 is essentially like that in Scelidosaurus and Echinodon. The 

 expanded crown is divided into a basal and apical portion ; 

 the marginal serrations of the latter are coextended with the 

 increased thickness of the part into small lamellse, themselves 

 more minutely dentate. The middle longitudinal rising of the 

 enamel, which in Echinodon is stronger on the outer side of the 

 upper teeth and on the inner side of the lower teeth, is exclu- 

 sively developed, as the "primary ridge" on the corresponding- 

 aspects of the teeth of the upper and lower jaws in Iguanodon. 

 In the teeth of the young Iguanodon, the primary ridge is 

 median and well-marked, and in the unworn tooth forms or 

 terminates at the apex of the crown, increasing its resem- 

 blance to the Echinodont type of tooth. The difference of 

 dental structure between Echinodon and Iguanodon is of the 



* From the Mid-Oolitic formation, called "Forest Marble," near Bradford, 

 Wilts. See my Odontography, p. 291, pi. 75a, fig. 7. 



