WEALDEN FORMATIONS. 11 



convexity transversely. The tlienal surface is flattened, but irregular ; an oblique groove 

 extends from the radial end of the proximal surface for about an inch onward toward 

 the ulnar side ; this groove, 4 lines in breadth, seems to be natural ; the clay matrix 

 could easily be picked out of it. Beyond the groove the short thenal surface is moderately 

 smooth and slightly concave ; a pair of hemispherical tuberosities project near the distal 

 articulation, and are continued into the tuberosities on each side of that surface. The 

 form of the surface is trochlear, that is, concave transversely, convex ancono-thenally ; 

 feebly defined in both directions. The breadth is 1 inch 3 lines; in the opposite 

 diameter 10 lines. The well-defined anconal border projects a little above the level of the 

 corresponding surface of the shaft; the breadth of the shaft at its middle is 1 inch 3 lines. 



To the well-defined smooth trochlear surface of the above phalanx is adapted a surface 

 of corresponding size, shape, and smoothness at the proximal end of a phalanx, 1 inch 3 

 lines in length, 1 inch 4^ lines across that end (PI. Ill, it, 2). The breadth of the distal 

 articulation of this phalanx is 1 inch 2 lines ; its ancono-thenal diameter is 6 lines, that of 

 the proximal surface being 9 lines. Thus, the shape of this phalanx is subquadrate and 

 subsphenoid ; the apex of the wedge being cut off, so to speak, to form the distal joint. 

 The upper surface of the short shaft is smooth, convex transversely, concave lengthwise. 

 The under surface is flat, rough, and irregular, and is continued into rough prominences 

 on each side of the shaft. 



To the distal articular surface of the above phalanx is adapted the proximal one of the 

 present (ib., n, 3, 3 a, 3 b), which is terminal, ending in a rough, obtuse, thickened border 

 (3 b) ; the breadth exceeds the length in a greater degree in this than in the preceding 

 phalanx; it equals 1 inch 3 lines, the length of the bone being 10 lines. The greatest 

 ancono-thenal diameter of the proximal end is 9 lines, while that of the articular surface 

 is but 6 lines ; there is no trace of attachment for the claw. The non-articular surface 

 of this obtusely wedge-shaped phalanx indicates by its roughness that it was imbedded 

 in a callous sheath of the integument. 



Thus we have evidence that the second digit of the fore-foot of the Iguanodon had 

 three phalanges supported by a metacarpal ; that it much exceeded in length the pollex 

 or first digit, and that it was of less breadth, though with greater ancono-thenal thickness 

 of the proximal phalanx. 



The entire length of the four bones of the second digit is 10 inches 6 lines. 



The metacarpal of the third or ' medius ' (ib., in, m) digit is 6 inches 9 lines in length ; 

 the ancono-thenal exceeds the transverse diameter, except at the distal articulation, where 

 the two are equal ; the bone is most compressed laterally at the proximal end, which is 

 strongly convex for being wedged or received into a groove-like cavity of the os magnum. 

 The ancono-thenal diameter at this end of the bone is 2 inches ; the transverse diameter 

 at the anconal part is 1 inch 3 lines, but narrowing towards the thenal end. The radial 

 side of the bone has a roughened tract, narrowing forward, and of the same extent as 

 that on the contiguous surface of the second metacarpal ; but it deviates from flatness at 



