The bones of the fore limb, generally, correspond in length with the similar parts of the hind limb. The upper arm bone corresponds with the upper leg bone, and the fore-arm bone is as long as the fore-leg bone; then differences begin. The bones which correspond to the back of the hand in man, termed the metacarpus, are variable in length in Pterodactyles--sometimes very long and sometimes short. The wing metacarpal bone is always stout, and the others are slender. The extremity of the metacarpus was applied to the ground. Three small digits of the hand are developed from the three small metacarpal bones, and terminate in large claws. The great wing finger was bent backward, and only touched the ground where it fitted upon the wing metacarpal bone. It appears sometimes to have been as long as the entire vertebral column. Owing to the circumstance that the joint in the arm in Pterodactyles was not at the wrist as among birds, but between the metacarpus and the phalanges, it follows that the fore limb was longer than the hind limb when the metacarpus was long; but the difference would not interfere with the movements of the animal, either upon four feet or on two feet, for in bats and birds the disproportion in length is greater. HUMERUS OR UPPER ARM BONE The first bone in the fore-arm, the humerus, is remarkable chiefly for the compressed crescent form of its upper articular end, which is never rounded like the head of the upper arm bone in man, and secondly for the great development of the external process of bone near that end, termed the radial crest. Sir Richard Owen compared the bone to the humerus of both birds and crocodiles, but in its upper articular end the crocodile bone may be said to be more like a bird than it is like the Pterodactyle. In flying reptiles the articular surface next to the shoulder-girdle is somewhat saddle-shaped, being concave from side to side above and convex vertically, while most animals with which it can be compared have the articular head of the bone convex in both directions. A remarkable exception to this general rule is found in some fossil animals from South Africa, which, from resemblance to mammals in their teeth, have been termed Theriodonts. They sometimes have the head of the bone concave from side to side and convex in the vertical direction. To this condition Ornithorhynchus makes a slight approximation. The singular expansion of the structure called the radial crest finds no close parallel in reptiles, though Crocodiles have a moderate crest on the humerus in the same position; and in Theriodonts the radial crest extends much further down the shaft of the humerus. No bird has a radial crest of a similar kind, though it is prolonged some way down the shaft in Archæopteryx. In Pterodactyles it sometimes terminates outward in a smooth, rounded surface, which might have been articular if any structure could have articulated with it. There is also