WEALDEN DINOSAURS. 517 



The reverse of all these conditions is seen in the bones of the hind limbs of the 

 Iguanodon and other Dinosaurian reptiles. 



If one takes the pleasure of speculating on the genesis of Didtis or Dinomis, guiding 

 or reining the roaming fancy by facts, the geographical limitation of such ornithicnitoid 

 species, and their primitive association exclusively vpith creatures of which they could have no 

 dread, suggest the more obvious and intelligible hypothesis of derivation from antecedent 

 birds of flight, whose wings they still show more or less aborted, according to BufFon's 

 principle of transmutation by degeneration, — with a progressive predominance of the legs 

 over the wings, ultimately resulting in a maximization of the terrestrial and abortion of 

 the aerial instruments of locomotion. 



Mandible and Mandibular Teeth (' Dinosauria,' Plates 49, 50). 



The dentary element of the right mandibular ramus of the young Iguanodon 

 {Dinosauria, Plates 16, 17), discovered in the Wealden of Stammerhara, near Horsham, 

 Sussex, by G. B. Holmes, Esq., demonstrated the fact that the sculptured surface of the 

 crown in the teeth of the lower jaw was turned inward, the smooth surface outward, 

 toward which aspect the entire tooth was moderately bent. Moreover, the alveoli in that 

 jaw showed eighteen teeth to be the number supported in a close-set series and working 

 position in the dentary element (««/e, p. 296). 



The portion of mandible obtained by S. H. Beckles, Esq., from the locality of the 

 limb-bones above described, is also the dentary element of the right ramus, of which a 

 figure of the inner side is given in Dinosauria, Plate 49. On this surface the crowns of 

 seven teeth nearly risen into place are seen ; the worn crown and fang of a few of the 

 preceding generation of teeth have been preserved, and the summits of the crown of a few 

 teeth of a third set in succession is seen in the interspaces of the more developed teeth 

 of the second set. 



The length of the portion of mandible here preserved is eighteen inches ; that of the 

 corresponding part of the mandible of the Iguanodon discovered by Captain Brickenden 

 in the Wealden of Filgate (Z)/»osa«n«, Plate 18) measures 20 inches. It is probable, 

 therefore, that Mr. Beckles' specimen had nearly attained the full average size of the 

 great herbivorous reptile. 



The antero-posterior breadth of the teeth rising into place averages 9 lines ; the largest 

 mandibular teeth of I(^uanodon {Dinosauria, Plate 45) give 1 inch in the same dimen- 

 sions. The crown-germs of the teeth in the Stammerham jaw {Dinosauria, Plate 10) 

 average 6 lines ; we thus learn that each successive series of teeth had an increase of 

 size corresponding in a general degree with the growth of the jaw. 



The subject of fig. 1, Plate 49, shows at its interior or symphysial end the abrupt slope 

 downward of the short, edentulous, compressed part, which curves inward to meet the 

 corresponding part of the opposite ramus at a short symphysis, extending along an hori- 



