The terminal claw bones are unlike the claws of Birds or Reptiles. They are compressed from side to side, and extremely deep and strong, with evidence of powerful attachment for ligaments, so that they rather resemble in their form and large size the claws of some of the carnivorous fossil reptiles, often grouped as Dinosauria, such as have been termed Aristosuchus and Megalosaurus. In the hand of the Ostrich the first and second digits terminate in claws, while the third is without a claw. But these claws of the Ostrich and other birds are slender, curved, and rather feeble organs. In the Archæopteryx, a fossil bird which agrees with the Pterodactyles in retaining the separate condition of the metacarpal bones and in having the same number of phalanges in two of the fingers of the fore limb, the terminal claws are rather more compressed from side to side, and stronger than in the Ostrich, but not nearly so strong as in the Pterodactyle. The Archæopteryx differs from the Pterodactyle in having no trace of a wing finger. The first metacarpal bone is short, as in all birds; and the first phalange scarcely lengthens that segment of the first digit of the Bird's hand to the same length as the other metacarpal bones. It therefore was not bent backward like the first digit in Pterodactyles. The wing finger, from which the genius of Cuvier selected the scientific name—Pterodactyle—for these fossils, yields their most distinctive character. It is a feature which could only be partly paralleled in the Bat, by making changes of structure which would remove every support to the wing but the outermost digit of that animal's hand. In the Bat's hand the membrane for flight is extended chiefly by four diverging metacarpal bones. There are only two or three phalanges in each digit in its four wing fingers. In Pterodactyles the metacarpal bones are, as we have seen, arranged in close contact, and take no part in stretching the wing.
FIG. 46. METACARPUS AND DIGITS OF THE HAND IN BIRDS WITH CLAWS
THE WING FINGER
In Birds there is nothing whatever to represent the wing finger of the Pterodactyle, for it is an organ external to the finger bones of the bird, and contains four phalanges. The first phalange is quite different from the others. Its length is astonishing when compared with the small phalanges of the clawed fingers. The articular surface, which joins on to the wing metacarpal bone, is a concave articulation, which fits the pulley in which that bone ends. The pulley articulation admits of an extension movement in one direction only. Many specimens show the wing finger to be folded up so as to extend backward. The whole finger is preserved in other specimens straightened out so as to be in line with the metacarpus. This condition is well seen in Professor Marsh's specimen of Rhamphorhynchus, which has the wing membrane preserved, in which all bones of the fore-arm metacarpus and wing finger are extended in a continuous curve. The outer surface of the end of the first bone of the wing finger overlaps the wing metacarpal, so that a maximum of strength and resistance is provided in the bony structures by which the wing is supported. There is, therefore, in flight only one angular bend in the limb, and that is between the upper arm bone and the fore-arm.
An immense pneumatic foramen is situate in a groove on the under side of the upper end of the first phalange in Ornithocheirus, but is absent in specimens from the Kimeridge clay. This bone is long and stout. It terminates at the lower end in an obliquely truncated articular surface. Specimens occur in the Cambridge Greensand which are 2 inches broad at the upper end and nearly 1½ inch wide at the lower end. An imperfect bone from the Chalk is 14½ inches long. The bones are all flattened. Specimens from the Chalk of Kansas at Munich are 28 inches long. The second phalange is concave at the upper articular end and convex in the longer direction at the lower end. The articular points of union between the several phalanges form prominences on the under side of the finger in consequence of the adjacent bones being a little widened at their junction. It should be mentioned that there is a proximal epiphysis or separate bone to the first phalange, adjacent to the pulley joint of the metacarpal bone, which is like the separate olecranon process of the ulna of the fore-arm. It sometimes comes away in specimens from the Chalk and Cambridge Greensand, leaving a large circular pit with a depressed narrow border. On the outer side of this process is a rounded boss, which may possibly have supported the bone, if it were applied to the ground with the wing folded up, like the wing of a Bat directed upward and backward at the animal's side.
The four bones of the wing finger usually decrease progressively in length, so that in Rhamphorhynchus, in which the length of the animal's head only slightly exceeds 3½ inches, the first phalange is nearly as long, the second phalange is about 3¼ inches, the third 2¾ inches, and the fourth a little over 2 inches. Thus the entire length of the four phalanges slightly exceeds 11 inches, or rather more than three times the length of the head. But the fore-arm and metacarpus in this type only measure 3 inches. Therefore the entire spread of wings could not have been more than 2 feet 9 inches.
The largest Ornithosaur in which accurate measurements have been made is the toothless Pterodactyle Ornithostoma, also named Pteranodon, from North America. In that type the head appears to have been about 3 or 4 feet long, and the wing finger exceeded 5 feet; while the length of the fore-arm and metacarpus exceeded 3 feet. The width of the body would not have been more than 1 foot. The length of the short humerus, which was about 11 inches, did not add greatly to the stretch of the wing; so that the spread of the wings as stretched in flight may be given as probably not exceeding 17 or 18 feet. A fine example of the wing bones of this animal quite as large has been obtained by the (British Museum Natural History). Many years ago, on very fragmentary materials, I estimated the wings in the English Cretaceous Ornithocheirus as probably having a stretch of 20 feet in the largest specimens, basing the calculation partly upon the extent of the longest wings in existing birds relatively to their bones, and partly upon the size of the largest associated bones which were then known.