ROOM III. UPPER JAW OF THE IGUANODON. 251 



five feet four inches as the length of the skull of the Iguanodon to which 

 the fossil belonged ; but as the brain and the organs of sense would 

 probably bear a less proportion to the whole bulk in these gigantic 

 saurians than in the small species of existing Lizards, we may infer 

 a diminution in the absolute size of the head, corresponding with the 

 abbreviation and contraction of the cranium ; and the length in the 

 adult Iguanodon would probably average about four feet. 



" The breadth of the fragment is uniform; in front it is rounded off 

 externally, and exhibits the oblong terminal irregular surface for articu- 

 lation with the intermaxillary bone by which it appears to have been 

 overlap t. The large infra-orbital canal opens at the junction of the 

 posterior and middle third, and midway between its margins passes into 

 a broad and deep and sigmoid groove, which curves inwards as it advance?, 

 so as nearly to reach the inner edge in the centre of its course, where it 

 gives off a retrograde furrow extending over the internal margin. 



" The infra-orbital canal, which is eight lines wide behind and four 

 lines high, bends inwards as it retrogrades from its anterior opening. 

 The inner surface is only four lines from the nasal aspect of the frag- 

 ment behind, so that after a course of a few inches, it would have 

 emerged on the floor of the nasal cavity. The roof is incised obliquely 

 outwards, and the inner portion of it extends forwards to the retrograde 

 groove. The portion of the external surface of the alveolar process that 

 remains, slopes inwards, and exhibits no traces of vascular foramina." 



From the almost entire destruction of the inner walls of 

 the alveolar furrow, deep transverse grooves are the only indi- 

 cations of the dental sockets. As the fangs of the teeth of 

 the upper jaw were more curved than in the lower series, 

 their implantation presented a corresponding modification, as 

 is the case in the dental organs of certain existing Monitors ; 

 hence the width of the alveolar space is greater than in the 

 lower jaw. 



Distinctive characters of the Upper and Lower Teeth. 

 Although the peculiar characters which distinguish the teeth 

 of the Iguanodon from those of all other animals were satis- 

 factorily established from the numerous detached specimens 

 that had come under my observation, yet as the mode in 

 which the teeth were implanted in the jaws was then un- 

 known, no attempt was made to ascertain the dextral or 

 sinistral position of the isolated teeth, nor to separate the 

 lower from the upper series, and thus determine the dental 

 arrangement by which the jaws of this colossal reptile were 

 invested with the functions of those of the existing herbi- 

 vorous mammalia. To ascertain these important questions it 

 became necessary to institute a rigorous examination and 

 comparison of all the teeth of the Iguanodon to which we 



