518 BRITISH FOSSIL REPTILES. 



zontal surface, parallel with the straight lower border of the mandible. The smooth canal 

 thus formed above the symphysis indicates a relation of facility in regard to the move- 

 ments of protrusion and retraction of a long, cylindrical, muscular tongue, probably used, 

 like that of the Giraffe and Megatherium, for the prehension of the vegetable substances 

 selected by the Iguanodon for food. 



The commencement of the coronoid process, contributed by the dentary, is the same 

 in extent as that shown in the younger Iguanodon's jaw {Din., Plate 16, a, /), and 

 indicates the position of the suture of the dentary with the surangular element. 



The surface of the tooth-crowns here exposed show the subniedian pi-imary vertical 

 ridge {a), which, in detached teeth, indicates the hinder border of the crown by its prox- 

 imity thereto. The secondary ridge [b) is faintly marked, but is best shown in the two 

 hindmost teeth. The anterior lamello-serrate border describes the usual convex curve ; 

 the posterior border being almost straight or slightly concave along its chief extent. The 

 dental characteristics of Iguanodon MantelU, as illustrated in previous plates {I)ui., 

 Plates 23, 45), are well maintained. The secondary ridge is, however, less developed 

 than in the larger teeth of older Iguanodous. The alveolar border here, as in the smaller 

 jaw, describes a gentle sigmoid curve in the transverse direction, the convexity being 

 inward in the hinder two thu'ds, then straight or slightly concave to the commencement 

 of the symphysial slope. 



In the inwardly convex part of the alveolar tract the teeth are placed ' en echellon ;' the 

 fore-and-aft plane of the anterior tooth {Bin., Plate 49, fig. \,d), if carried back, would pass 

 outside the succeeding tooth (ib., b), and the crown of this stands in like relation to the 

 next tooth behind (ib., c). Thus, when fully in place, the crowns slightly overlap in the 

 lower as in the upper jaw {ante. Bin., Plate 45, fig. 2), and thus, eighteen teeth may 

 range along an alveolar tract, which, if each tooth stood clear of the next, Avould not 

 support more than fourteen. Room is also got for the full number along the working 

 line by a certain alternation in the degree of attrition, as is well exemplified in the 

 portion of mandible of a younger Iguanodon next to be described (Plate 50). 



I am indebted to A. J. Hogg, Esq., for the opportunity of examining and figuring 

 this instructive specimen. It was discovered in the hard limestone, locally known as the 

 "Under Peather," which is situated from four to five feet below the accumulation of 

 shells of Ostrea dislorfa, called the " Cinder Bed," in the Middle Purbecks. 



A reference to p. 22, fig. 4, of my ' Monograph on the Fossil Mammalia of the 

 Purbeck Formations, British Mesozoic Mammals' (Palseontographical Society, vol. xxiv, 

 issued for 1870), will show the position in the Middle Purbeck series in which the 

 present interesting evidence of the Iguanodon was entombed. It is the first example of 

 that genus, to my knowledge, from the Purbeck series. 



