12 FOSSIL REPTILIA OF THE 



plate (Tab. VI, fig. 2, 32), applied to the inner side of the dentary. The dentary 

 (33) is a very powerful bone, with the outer surface divided into an upper and 

 lower facet by a longitudinal ridge paralleling that of the upper jaw. The ridge 

 (Tabs. IV and V, 33), commencing near the base of the coronoid process, descends, 

 describing a slight curve to the middle of the outer surface of the dentary. 

 Below the ridge the bone is convex, above it is concave ; the lower facet has 

 the kind and degree of roughness observable on the exposed surface of most of 

 the cranial bones ; the upper facet has a smoother surface, corresponding in that 

 respect with the surface below the ridge of the maxillary. 



The foregoing character of the lower jaw has, hitherto, been observed only in 

 a fossil one, which has been referred to the Dinosaurian order; by Mantell,* 

 originally to Iguanodon, and afterwards, when it had been shown to be more 

 probably part of the Hylaosaurus,~\ to a genus which he called Regnosaurus.'l In 

 this portion of the ramus of the mandible (No. 422, Reptilian Fossils, British 

 Museum), the outer surface of the dentary is divided into an upper and lower 

 facet by a longitudinal ridge, which, commencing near the upper margin, probably 

 at the base of a coronoid rising, descends as it advances to midway between the 

 upper and lower border. It is, however, more obtuse than in Scelidosaurus, but the 

 upper facet presents a like smoothness and vertical concavity. § In size the speci- 

 mens closely correspond, and also in the close arrangement of the series of teeth. 

 But these were relatively smaller and more numerous in the Wealden fossil ; for 

 whereas in Hyleeosaurus ten teeth, or their sockets, occupy an extent of 1 inch 8 

 lines of the alveolar border, the same extent includes only seven and a half teeth or 

 sockets in Scelidosaurus. In this genus, moreover, the ramus of the mandible 

 presents a curve convex downwards, to about the same degree as the opposite 

 curve is presented by the corresponding part of the jaw of Hylaosaurus, in which 

 this peculiar bend is noticed by me in a former monograph. || In the mandible 

 of Scelidosaurus a ridge, corresponding, perhaps, to the lower ridge in Hylaosaurus 

 is situated further back and higher up upon the surangular ; and the facet, 

 concave vertically between the lower ridge and the beginning of the upper 

 ridge, ^1 is peculiar to the mandibular fragment referred to the Hyl<sosaurus. Thus, 

 with corresponding Dinosaurian character, imparting robust strength to the man- 

 dible, there are well-marked generic distinctions in the specimens here compared, 

 both in the conformation of the jaws and teeth.** 



The mandibular rami of Scelidosaurus describe a slight, but graceful, sigmoid 



* « Wonders of Geology,' 1838, vol. i, p. 393. 



f "Report on British Fossil Reptiles," 'Trans, of Brit. Association,' 1841, p. 120. 



% 'Philos. Trans.,' 1848. 



§ See tab. viii, fig. 5, 'Monograph on Wealden Reptilia' (1856). 



I! lb., p. 19. 1f lb., tab. viii, figs. 1 and 4. ** lb., tab. viii, figs. 6—9. 



