LIASSIC DINOSAURS. 97 



dentary (Pis. 45, 46, 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, 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 Igimnodon, and afterwards, when it had been shown to be more 

 probably part of the Eylceosaurus,^ to a genus which he called Begnosaurus? In 

 this specimen 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 Hke smoothness and vertical 

 concavity.* In size the specimens 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 Hijlceosaurus 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 Hiflceosaurus. in which this peculiar bend is noticed in Vol. i, 

 p. 366. In the mandible of Scelidosaurus a ridge, corresponding, perhaps, to 

 the lower ridge in Eylmosaunhs, 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, is peculiar to the mandibular fragment referred to 

 the Hijlceosaurus. Thus, with corresponding Dinosaurian character, imparting 

 robust strength to the mandible, 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 sigmoid curve from the 

 angle forward, horizontally, at first concave, then convex, towards the median line, 

 where they meet without blending at the part fractured. It is not probable that 

 the symphysis would be much prolonged beyond this point. The degree of 

 convergence of the contour lines of the whole skull, both median and lateral, with 



1 ' Wonders of Geology,' 1838, vol. i, p. 393. 



2 " Eeport on British FoFsil Keptiles," 'Trans, of Brit. Association,' 1841, p. 120. 



3 ' Philos. Trans.,' 1818. * Vol. ii, pi. 39, fig. 1. 



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