20 FOSSIL REPTILIA OF THE 



and at the part where the inner wall has been least mutilated, it nearly completes the 

 socket, and incloses the long and slender fang of the tooth. Whence, I conclude, 

 that the entire jaw of the extinct reptile would have exhibited a series of true sockets, 

 with oblique outlets, not depressions merely, as in the present mutilated fragment ; 

 and that it would have agreed with the Megalosaurus in presenting the sub-thecodont 

 mode of implantation of the teeth. 



The crowns of all the teeth are broken off ; the small sockets of reserve, exposed 

 at the inner side of the base of the old sockets, do not contain any evidence of the 

 species to which this fossil has belonged. 



In my ' Odontography,'* I adopted the opinion of Dr. Mantellf respecting the 

 present fossil, viz., that it belonged to a young Iguanodon; but subsequent considera- 

 tions^ induced me to refer it to the same species of extinct reptile as the teeth 

 (T. VIII, figs. 6—9) belonged to. 



Since the publication of my ' Reports on British Fossil Mammalia,' the lower jaw of 

 the Iguanodon has been discovered, and leaves no room for doubt as to the generic and 

 specific distinction of the present fossil. In the portion of jaw in question (T. VIII, 

 figs. 2 and 3) there are eighteen alveoli in an extent of three inches : in the lower jaw 

 of a young Iguanodon of the same size, there are but nine alveoli in the same longitu- 

 dinal extent ; whilst in three inches of the dentary border of the mandible of an older 

 Iguanodon, there are but four alveoli. The form of the alveoli, as I had inferred from 

 the known shape of the teeth of the Iguanodon, differs from that of the alveoli in the 

 portion of jaw figured in T. VIII ; but those alveoli accord with the shape of the fangs 

 of the teeth next to be described. 



* Part II, 1839, p. 248. f 'Wonders of Geology,' vol. i, p. 393. 



X " In the absence of this characteristic part of the tooth, an element in guiding our choice between 

 the Iguanodon and Hylseosaur is given by the breadth of the interspaces of the sockets ; these must bear 

 relation to the breadth of the crowns of the teeth, if we suppose that they were in contact throughout the 

 series, as in Lacertians. Now, the teeth of the Iguanodon, and those which I have referred to the 

 Hylaeosaur, differ in a marked degree in the breadth of the crown. The complicated and expanded 

 crown of the Iguanodon's tooth is supported on a narrower stem ; and the stems or fangs, if the crowns 

 were in contact without overlapping, must have been separated by interspaces of proportional breadth, viz., 

 twice their own breadth ; but the thickness of the crown of the tooth of the Iguanodon renders it very 

 unlikely that they did overlap each other. Now, the crowns of the teeth of the Hylaeosaur are expanded 

 to such an extent as, if in contact, to require an interspace of the fangs, not broader than the fangs them- 

 selves ; and the interspaces of the fangs in the fragment of jaw under consideration correspond with 

 crowns of this breadth. The fangs of the teeth in the Iguanodon are conical, and more or less angular ; in 

 the teeth presumed to belong to the Hylaeosaur the fangs are cylindrical ; the sockets in the present 

 fragment correspond with the latter form." (Report on British Fossil Reptilia, in the • Reports of British 

 Association,' 1841, p. 110.) 



