ON BRITISH FOSSIL REPTILES. 139 



mutilated state, but on examining the posterior part of the shaft, where on the 

 internal side one might expect to meet with some remains of the base of the 

 trochanter, I find the surface of the bone concave, and it diverges much more 

 than I should suppose it would do if it had merely been continuous with the 

 returning surface from the anterior part of the bone, if there had been no 

 trochanter interposed to disturb the otherwise greater rotundity of the shape. 



" This leads me to suppose that it once had one, and that it probably might 

 ha,ve been formed like that in Nos. 1 and 2 in the British Museum. If they 

 were not the bones of distinct animals, this might perhaps have been the 

 case." Dated Horsham, Nov. 2nd, 1841. 



The characters of the articular extremities of the femur which are obscured 

 by the mutilated condition of the large specimen, are beautifully shown in 

 the femur of a young IguanodoH, obtained from a pit near Rusper, four miles 

 north of Horsham. The rounded portion of the head extends inwards ; it is 

 indented at its anterior part by the commencement of a longitudinal broad 

 channel, which extends down uj)on the shaft : the articular surface is not con- 

 fined to the inwardly produced head, but extends over the whole proximal 

 horizontal surface of the femur, expanding as it approaches the outer part of 

 the head. The articular surface is circumscribed by a well-defined linear 

 groove, which separates it from the longitudinal striated surface of the shaft 

 of the bone. At the posterior and external angle of the articular proximal 

 end of the bone, a longitudinal column, separated by a longitudinal groove 

 from the main shaft of the bone, falls into that shaft a little lower down the 

 distal end : here the shaft expands and becomes flattened from before back- 

 wards. The distal end is characterized by a deep and narrow anterior longi- 

 tudinal groove, situated not quite in the middle, but nearer the internal con- 

 dyle : there is a corresponding longitudinal groove on the posterior part of 

 the distal end, which is wider than the anterior one, and in the middle of the 

 bone, separating the two condyles, but inclining beneath, and, as it were, un- 

 dermining the backward projecting part of the internal condyle ; this is much 

 more prominent than the external one, which is traversed or divided by a 

 narrow longitudinal fissure. The articular surface is irregular and tubercu- 

 late. In. Lines. 



The lateral diameter of proximal end 2 8 



The lateral diameter of distal end 3 



Antero-posterior diameter of outer part of proximal end . 2 

 Antero-posterior diameter of outer part of internal condyle 2 3 



The femur of the Iguana differs as widely from that of the Iguanodon as 

 does that of the Monitor or any other Lacertiau reptile. The forms of the 

 head and trochanter of the femur of the Iguana are just the reverse of those 

 in the Iguanodon. The head of the femur in the Iguana is flattened from side 

 to side, and its upper convex surface is extended from before backwards, ma- 

 king no projection over the gentle concave line leading from its inner surface 

 down to the inner condyle. In the Iguanodon the head is rounded and rather 

 compressed from before backwards ; and is produced, as in Mammals, over 

 the inner side of the shaft. 



In the Iguana the trochanter is compressed from before b.ackwards, and is 

 separated by a wide and shallow groove from the oppositely compressed head : 

 in the Iguanodon the trochanter is singularly flattened from side to side, and 

 is applied to the outer side of the thick neck, from which it is separated by a 

 deep and narrow fissure. The Iguana has no submedian internal process, and 

 its distal condyles are slightly divided by a shallow depression. 



The circumference of the femur of the Iguanodon very nearly equals one- 

 half its length : the circumference of the femur of the Iguana only equals one- 



