758 



to the fragmentary and water-worn condition in which the fossil re- 

 mains of terrestrial vertebrated animals occur in fluviatile deposits, 

 in consequence of these strata consisting of materials transported from 

 far-distant lands, nearly a quarter of a century elapsed before any 

 portion of the jaw with teeth was discovered. 



The most important of the fossils described in this memoir con- 

 sists of the anterior part of the right side of the lower jaw, which was 

 discovered a few wrecks since, in a quarry in Tilgate Forest, by Capt. 

 Lambart Brickenden, F.G.S., who with great liberality placed it at 

 the disposal of the author as the original investigator of the fossils 

 of the Wealden. 



This dentary bone, wdiich is eighteen inches long, is perfect in the 

 anterior part, but is broken at the hinder extremit}^ and retains five 

 or six inches of the coronoid bone : the length of the jaw to which it 

 belonged is estimated at four feet. It contains two successional teeth 

 in place, the fang of a third, and the alveoli or sockets for eighteen 

 or nineteen mature molars ; the entire number of teeth on each side 

 the lower jaw was about twentj''. 



The mature teeth, which, when abraded by use in mastication, 

 resemble the worn molars of herbivorous mammalia, appear to have 

 been arranged in a close-set series. The lower teeth had their ena- 

 melled striated face parallel with the alveolar plate, and fronting the 

 inside of the mouth : but the upper were placed in a reverse position, 

 that is, with the enamelled facet of the crown external; and the 

 teeth in the upper and lower jaws were arranged subaltemate or in- 

 termediate in relation to each other, as is the case in the ruminants. 



But a still more remarkable character presented by this specimen 

 is the peculiar construction of the anterior part of the lower jaw, 

 which forms the symphysis. This process, instead of being con- 

 tinued round the front of the mouth and beset with teeth, as in all 

 other saurians, is edentulous, and extends into a procumbent scoop- 

 like expansion, very analogous to the symphysial portion of the 

 lower jaw in the Sloths, and especially to that of the colossal extinct 

 Edentata — the Mylodons. Along the external surface of this dentary 

 bone there is a row of very large vascular foramina ; and the sym- 

 physis also is perforated with numerous openings for the passage of 

 blood-vessels and nerves sent oS from the great dental canal. The 

 unusual number and magnitude of these foramina indicate a great 

 development of the integuments and soft parts with which the bone 

 was inevsted, and also the large size of the under lip. 



The upper jaw, of which a considerable portion discovered by the 

 author is in the British IMuseum, confirms the inferences deduced 

 from the teeth and dentary bone of the lower maxilla. 



The author, with the able assistance of Dr. A. G. Melville, instituted 

 a comparison between all the teeth of the Iguanodon to which he could 

 obtain access, and those of recent saurians ; and the result of the 

 investigation is detailed. The new light shed on the structure and 

 functions of the dental organs, confirms, in every essential particular, 

 the inferences deduced by the author from the detached teeth alone, 

 in his memoir of 1825 ; and it also reveals an extraordinary devia- 



