6 FOSSIL REPTILIA OF THE 



expanding as it descends ; concave, but in a less degree, transversely ; with the inner 

 (tibial) side of greater extent. On both sides the articular border is slightly raised, 

 forming the lower boundary of the wide concavity for the attachment of the lateral 

 ligaments. 



The second phalanx (n 2) is broader than it is long, its extreme breadth 

 being 2 inches 4 lines. The proximal articular surface, with its concavity and con- 

 vexity the reverse of those of the surface on which it plays, is triangular, with the 

 angles largely rounded off. The under surface of this phalanx is somewhat flattened ; 

 the upper surface is contracted ; the distal trochlea, very convex vertically, is flat 

 transversely, at its upper half, slightly concave below ; the modification resembling 

 that of the phalanx supporting the unequal one in the other toes. 



The third phalanx (n 3), which supported the claw, presents an oblique basal 

 articular surface, flattened transversely and produced backwards above; slightly 

 convex transversely below. The unequal part is sub-depressed, obtuse, obliquely 

 bent downwards and outwards, but in a slight degree : the base of the bone is notched 

 at each side, where the vascular canals relating to the growth of the claw commence ; 

 they impress the upper and lateral parts of the bone, which is A\ inches in length. 



The proximal phalanx of the middle toe (T. I and III, m l), answering to the third 

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

 4| 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 T. I, 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. 



