30 FOSSIL REPTILIA OF THE CRETACEOUS FORMATIONS. 



is most marked along the apical half, but is here much less prominent than in the 

 upper teeth ; it divides the crown into two unequal areae, the front one 

 (fig. 5, m, a, c) being at its broadest part nearly twice the breadth of the back one 

 (ib., a, d). The front area is pretty equally subdivided by a low, secondary, 

 longitudinal ridge, b, each division being feebly concave across. The angle 

 between the entire (fig. 11, i) and serrated (fig. 11 c,) parts of the borders of the 

 crown is more marked than in the upper teeth ; the basal part of the posterior 

 border (fig. 11, d) seems as if it were pushed inward and forward by the crown of 

 the succeeding and less developed tooth. The anterior serrated border (fig. 5, n,c) 

 is at first straight, then describes a bold, convex curve as it approaches the apex. 

 The posterior border (ib., d) passes almost to that apex in a straight line before it 

 is rounded off to the obtuse summit, where the primary ridge terminates. At the 

 fore part of the tooth (fig. 9) the fang is convex, and the basal half of the crown 

 shows a lanceolate depression, slightly concave across. The back part of the 

 tooth (fig. 8) shows a longer, shallow depression, s, extending over the upper half 

 of the fang and lower third of the crown. The inner or longitudinally convex 

 side of the narrow fang, in worn teeth, is sharply excavated, even to expose the 

 pulp-cavity, by the crown of the successional tooth (figs. 13 and 14, p). 



The apex of the crown of a young successional tooth is shown at r, on the 

 inner side of the tooth j^ in fig. 5. The remnant of the fang and alveolar 

 depressions of the old and shed teeth are shown at t, t, on the outer side of the 

 succeeding teeth, in fig. 7. Both are from the lower jaw. 



The upper part of the outer alveolar wall of the mandible bends out, so as to 

 be concave vertically ; its border is more deeply crenate than in the upper jaw. 

 A vascular canal runs about an inch and a half beneath it, from the oblique 

 orifices open upon the outer surface of the mandible. 



Figs. 10 — 14, in Tab. VII, from the dental series of the same individual, dis- 

 covered in the Greensand of Black Gang Chine, exemplify different degrees of de- 

 struction of the tooth by abrasion and absorption. Fig. 10 is an unworn tooth from 

 the fore part of the lower jaw. Figs. 11 — 14 show the size of the majority of the 

 teeth. In figs. 13, 14 the letter p marks the cavity caused by pressure of the new 

 or successional tooth ; in fig. 14 it has laid open the pulp-cavity of the old tooth. 



Fig. 15 shows the inner side, and fig. 16 the fore part, of a mandibular tooth 

 of a young Iguanodon, from the Upper Greensand near Cambridge. The inner 

 side of the fang shows the excavation due to the pressure of the successional 

 tooth (;), fig. 15). Fig 17 shows the outer and inner sides of a smaller tooth 

 of an Iguanodon, from the same formation and locality. All the evidences 

 of Iguanodon which have yet reached me therefrom indicate a small size ; but 

 whether this may relate to the immaturity of the individual, or to a small variety, 

 I am uncertain. 



