PEESUMABLT THAT OP KHJANODON - MANTELLI. 747 



remains of the cartilaginous rod upon which the bony elements of 

 the jaw were laid down. All that part of the vertical plate which 

 underlies and supports the tympanic surface is stout and sutural. 

 The lower border of the vertical plate in front, where it is broken 

 through, is 1 to 1^- inch wide ; and behind it tapers to nothing at 

 the hinder end of the jaw. In its whole length this border is longi- 

 tudinally grooved by sutural marks. 



The horizontal plate bearing the tympanic surface, viewed from 

 above, has a roughly triangular outline (fig. 2 ; fig. 3 a). Its inner 

 border is almost straight : and for its anterior two thirds it projects 

 inwards beyond the vertical plate in a shelf-like manner. It meets 

 in front the anterior border, making with this an angle which, in 

 a less incomplete example, ran forward for about an inch in a 

 tongue-like form. The anterior border, rising and at the same time 

 acquiring a slight outward twist, becomes a highly inclined curved 

 sheet, which above, at its outer border, is separated by a shallow 

 groove from the upper lip of the vertical plate. 



The tympanic articular surface is hollow from side to side, and 

 also from before backward. Its greatest length is in the direction 

 of the axis of the jaw ; and in this direction, behind, a low median 

 rising imperfectly marks off an inner and an outer part. 



The above details will have made it apparent that the mandible 

 represented by this articular bone differed greatly from that of 

 Crocodilia, and in a less degree from those of extant lizards ; whilst in 

 some important features it resembled that of Hypsilophodon Foxii. 

 This, and the occurrence of the bone in relative abundance in the same 

 beds and localities which have yielded Iguanodon-mandibles, always 

 wanting the articular element, appear to me sufficient warrant for 

 (at least provisionally) referring this os articulare to the Iguanodon. 



