CRETACEOUS FORMATIONS. 29 



the shortest and straightest (fig. 2, </). A few, irregular, linear, minute, ridges 

 mark the enamel in both areae ; being more numerous, from three to five, in the 

 wider one, and not more than one or two of these extend from the base to the 

 apex of the crown; at the base they converge and sometimes unite as they descend. 



The fore part of the tooth is slightly hollowed at the basal half of the crown 

 (fig. 4, e) ; the fossa, which is elongated and concave transversely, gradually filling 

 up towards the apex ; below the middle of the crown, at the apical half, the fore 

 part of the crown (fig 1, e) is convex transversely. The hind part of the tooth 

 (fig. 4, /) is impressed by a longer, wider, and shallower depression, beyond 

 which it shows an oblique, rather flattened than convex, surface. The inner part 

 of the tooth, which is narrow in the fang (fig. 4, g), gradually expands upon the 

 crown to near the apex, where it again grows narrower ; at its broadest part 

 it is flattened or even a little concave transversely, but rounds off convexly into 

 the fore and hind parts of the crown (fig. 1, m)- 



The abraded surface of the crown is remarkably smooth and level ; it inclines 

 from before downward and backward, and more so from within downward and 

 outward in the upper jaw. 



The longitudinally convex and ridged part of the crown being external in the 

 upper teeth, and the position of the primary ridge determining the fore and hind 

 borders of the crown, a detached tooth may be at once referred to the right or 

 left maxillary bone. The germ of the successional tooth causes an excavation on 

 the inner, and generally towards the hinder, part of the base of the one in use. 



In a left upper tooth, with one fourth of the crown abraded, and projecting 

 1 inch 9 lines from the alveolar border, the crown of the successional tooth had 

 its apex on a level with that border, and on the inner and back part of this crown 

 was the thin shell of the apex of a third tooth in the successive series. 



The outer alveolar wall of the upper jaw is very thin at the outlet of the 

 sockets, and is a little produced at the intervals of the teeth ; it rapidly increases 

 in thickness towards the base of the sockets. 



The inner or palatal wall also thins off to a crenate edge; so much as is preserved 

 in the specimens examined was flat and smooth, as in fig. 1. The grinding sur- 

 face of the tooth (»2), of which one third of the apex had been worn away by mas- 

 tication, projected only about half an inch from the inner alveolar margin. 



The lower or mandibular teeth of Iguanodon have a broader crown, and 

 a fang less thick transversely to the jaw than the upper teeth ; they are more 

 curved lengthwise, the curvature being concave outward, contrary to that of the 

 upper teeth. The outer side of the tooth (fig. 6, tn, and fig. 11, o) is smooth and 

 convex from the fore (c) to the hind (d) border, its greatest breadth being opposite 

 the middle of the crown. The primary ridge, commencing at the enamelled base 

 of the inner and flatter part of the crown (fig. 5, m, a, and fig. 11, a), slowly rises, and 



