The Pterodactyles from the Solenhofen Slate which possess long tails have a series of characters which show affinity with the other long-tailed types. The jaws are much more slender. The orbit of the eye in Rhamphorhynchus is enormously large, and placed vertically above the articulation for the lower jaw. Immediately in front of the eye are two small and elongated openings, the hinder of which, known as the antorbital vacuity, is often slightly smaller than the nostril, which is placed in the middle length of the head, or a little further back, giving a long dagger-shaped jaw, which terminates in a toothless spear. The lower jaw has a corresponding sharp extremity. The teeth are directed forward in a way that is quite exceptional. Notwithstanding the massiveness and elongation of the neck vertebræ, which are nearly twice as long as those of the back, the neck is sometimes only about half the length of the skull.
All these long-tailed species from the Lithographic Stone agree in having the sternum broad, with a long strong keel, extending far forward. The coracoid bones extend outward like those of a Crocodile, so as to widen the chest cavity instead of being carried forward as the bones are in Birds. These bones in this animal were attached to the anterior extremity of the sternum, so that the keel extended in advance of the articulation as in other Pterodactyles. The breadth of the sternum shows that, as in Mammals, the fore part of the body must have been fully twice the width of the region of the hip-girdle, where the slenderer hind limbs were attached. The length of the fore limb was enormous, for although the head suggests an immense length relatively to the body, nearly equal to neck and back together, the head is not more than a third of the length of the wing bones. The wing bones are remarkable for the short powerful humerus with an expanded radial crest, which is fully equal in width to half the length of the bone. Another character is the extreme shortness of the metacarpus, usually associated with immense strength of the wing metacarpal bone.
The hind limbs are relatively small and relatively short. The femur is usually shorter than the humerus, and the tibia is much shorter than the ulna. The bones of the instep, instead of being held together firmly as in the Lias genera, diverge from each other, widening out, though it often happens that four of the five metatarsals differ but little in length. The fifth digit is always shorter.
The hip-girdle of bones differs chiefly from other types in the way in which those bones, which have sometimes been likened to the marsupial bones, are conditioned. They may be a pair of triangular bones which meet in the middle line, so that there is an outer angle like the arm of a capital Y. Sometimes these triangular bones are blended into a curved, bow-shaped arch, which in several specimens appears to extend forward from near the place of articulation of the femur. This is seen in fossil skeletons at Heidelberg and Munich. It is possible that this position is an accident of preservation, and that the prepubic bones are really attached to the lower border of the pubic bones.
Immense as the length of the tail appears to be, exceeding the skull and remainder of the vertebral column, it falls far short of the combined length of the phalanges of the wing finger. The power of flight was manifestly greater in Rhamphorhynchus than in other members of the group, and all the modifications of the skeleton tend towards adaptation of the animals for flying. The most remarkable modification of structure at the extremity of the tail was made known by Professor Marsh in a vertical, leaf-like expansion in this genus, which had not previously been observed ([p. 161]). The vertebræ go on steadily diminishing in length in the usual way, and then the ossified structures which bordered the tail bones and run parallel with the vertebræ in all the Rhamphorhynchus family, suddenly diverge downward and upward at right angles to the vertebræ, forming a vertical crest above and a corresponding keel below; and between these structures, which are identified with the neural spines and chevron bones of ordinary vertebræ, the membrane extends, giving the extremity of the tail a rudder-like feature, which, from knowledge of the construction of the tail of a child's kite, may well be thought to have had influence in directing and steadying the animal's movements. There are many minor features in the shoulder-girdle, which show that the coracoid, for example, was becoming unlike that bone in the Lias, though it still continues to have a bony union with the elongated shoulder-blade of the back.
FIG. 56. RESTORATION OF THE SKELETON OF RHAMPHORHYNCHUS PHYLLURUS
From the Solenhofen Slate, partly based upon the skeleton with the wing membranes preserved
