WEALDEN FORMATIONS. 3 



As a general rule, only the metapodials which bound or form the outer and the 

 inner sides of that segment of the foot have the proximo-lateral articular surface con- 

 fined to one side of the bone ; the intermediate metapodials show such surface on both 

 sides, for articulation with the contiguous metapodials. The metapodial (T. I, If, 

 and III, iv), which will presently be shown to be the outermost, had its outer side 

 rounded, and simply roughened for the implantation of ligamentous fibres ; the meta- 

 podial on the opposite side (ib., n) also presented a convexity toward that border of 

 the foot ; but a small part of the middle of that convexity is articulated with a slender 

 rudiment of a metapodial (ib., i), which forms the real boundary of that — the inner 

 side of the foot. The upper portion of this metapodial, which resembles the " splint- 

 bone" in the metapodium of the horse, has been fractured and partially dislocated before 

 the induration of the matrix; the lower portion of the bone is in its natural position, 

 and seems to have been anchylosed with the contiguous fully developed metapodium : 

 the extremity of this lower portion, however, is broken away ; so that, whether it 

 ended in a point, like the rudimental metapodials in the horse, or supported a 

 diminutive toe, like the metapodials of the spurious hoofs in the ox and musk-deer, 

 cannot be at present determined. 



As the fully developed toes which follow this rudiment have respectively three, 

 four, and five phalanges, the analogy of both the fore- and hind-foot of the Iguanas 

 and Monitors would indicate the small innermost metapodial (T. I, II, and III, i) to be 

 the rudiment of the first toe (pollex or hallux), and the three fully developed toes to be 

 the homologues of the second, third, and fourth toes of the feet in the Lizard tribe ; 

 the fifth toe being wholly suppressed in the Iguanodon. The analogy of the Croco- 

 dilian foot would lead to the same conclusion, since the second toe in that reptile has 

 three phalanges, and the third toe has four phalanges, whilst in the hind-foot the fifth 

 toe is suppressed. The fourth toe, however, in the Crocodilia differs from that in the 

 Lacertilia, in having only four phalanges, and usually wanting a claw. Hence it would 

 seem that, whilst the Iguanodon resembled the Crocodilia, as regards the hind-foot in 

 that order, in the suppression of the fifth toe, it resembled the Lacertilia in having the 

 fourth toe unguiculate, and with five phalanges: but it differs from both those Rep- 

 tilian orders in the suppression of the first toe, and its representation by a hidden 

 rudimental metatarsal, thus reducing the number of conspicuous and functional toes 

 to " three." 



The resemblance to the hind-foot of the Crocodilia in the suppression of the fifth 

 toe, and the resemblance of the third and fourth toes, in regard to their nearly equal 

 length, to those toes in the Monitor, render it most probable that the tridactyle foot 

 of the Iguanodon, here described, is a " hind-foot ;" but it cannot be assumed that 

 the fore-foot may not have been similarly modified. 



In the leading characteristics of the bony framework of the foot, whether fore or 

 hind, it is interesting to find that the Iguanodon manifests a combination of Croco- 



