378 BRITISH FOSSIL REPTILES. 



third in the Iguana, shows its increase chiefly in breadth and thickness ; its length is 

 4J inches. The proximal end, of a transversely oval form, is slightly and irregularly 

 concave ; its distal end is broader but less deep than that of the outer toe, and the 

 shape of the trochlea is more symmetrical ; the outer slightly exceeds the inner side 

 in extent. The increase in the transverse over the longitudinal and vertical diameters 

 is more marked in the second and third phalanges (m 2 and 3) of the middle toe ; the 

 latter phalanx shows the same flatness transversely, at the upper part of its distal 

 trochlea, as in the corresponding phalanx of the outer toe. This structure indicates 

 the next phalanx to have been an ungual one, resembling, as Mr. Beckles informed 

 me, in its general character, the long terminal phalanx in the adjoining toes. It is 

 indicated in outline in PI. 43, 4 in. 



All the five phalanges of the outer toe (iv 1, 2, 3,4, 5) are preserved; the entire 

 length of the toe is 8| inches, being rather shorter than the inner, but apparently 

 longer from the lower position of the metapodial bone (iv). In this proportion the 

 Iguanodon differed from existing Lizards, and resembled the Crocodiles. 



The proximal phalanx of the outer toe (iv 1), answering to the fourth in the 

 Iguana's foot, is 3 inches in length, with a subtrihedral body, one side turned to the 

 next toe, and one angle inwards and downwards. The proximal surface is flat ; the 

 distal one trochlear, but with the transverse concavity less deep than in the first 

 phalanx of the inner toe. The three succeeding phalanges (iv 2, 3, and 4) are similar 

 in character, but progressively decrease in size ; they are very short in comparison to 

 their breadth. 



The ungual phalanx (iv 5) is relatively more slender than in the inner toe : its 

 length is 4 inches; its basal breadth 1 inch 10 lines. The obliquity of the bone is 

 slight, and in the opposite direction to that of the inner toe. 



Should any rudimental or spurious claw have been supported by the metapodial of 

 the innermost digit (PI. 44, fig. 1, i), the development of which toe is so remarkably 

 arrested, it would probably present that form, and in regard to the fully grown 

 Iguanodon, that size, which characterises the claw-phalanx which has been mistaken 

 for the "horn" of the Iguanodon. It is probable that, in the fore-foot, the toe 

 answering to the innermost in the Iguana's foot was better developed than its homotype 

 in the hind-foot. 



Not far from where the foot-bones were found, the femur, tibia, and fibula, of the 

 same Iguanodon were extracted, — a circumstance which adds to the probability of 

 their belonging to the same limb. 



The modification of the present foot, whether of the fore- or hind-limb, of the 

 Iguanodon is unique, according to present knowledge, in the class Eeptilia. It 

 exhibits an adaptation to terrestrial progression, and the support of a weighty super- 

 incumbent trunk, akin to that which we observe in the tridact3'le foot of the heavy 

 perissodactyle Pachyderms, represented at the present day by the Rhinoceros and 

 Tapir. 



