(Cambridge Greensand)
The articular surfaces between the bodies of the vertebræ, in the neck, are transversely oval. The middle part of this articular joint is made by the body of the vertebra; its outer parts are in the neural arch. In front this surface is a hollow channel, often more depressed than in any other animals. The corresponding surface behind is convex, with a process on each side at its lower outer angles ([Fig. 25]). It is a modification of the cup-and-ball form of vertebral articulation, which at the present day is eminently reptilian. Serpents and Crocodiles have the articulations similarly vertical, but in both the form of the articulation is a circle. In Lizards the articular cup is usually rather wider than deep, when the cup and ball are developed in the vertebræ; it differs from the vertical condition in pterodactyles in being oblique and much narrower from side to side. Only among Crocodiles and Hatteria is there a double articulation for the cervical rib, though in neither order have rib or vertebra in the neck the bird-like proportions which are usual in these animals. Pterodactyles show no resemblance to birds in this vertebral articulation. A Bird has the corresponding surface concave from side to side in front, but it is also convex from above downward, producing what is known as the saddle-shaped form which is peculiarly avian, being found in existing birds except in part of the back in Penguins. It is faintly approximated to in one or two neck vertebræ in man. Professor Williston remarks that in the toothless Pterodactyles of Kansas the hinder ball of the vertebral articulation is continued downward and outward as a concave articulation upon the processes at its outer corners. There are no mammals with a cup-and-ball articulation between the vertebræ, so that for what it is worth the character now described in Ornithosaurs is reptilian, when judged by comparison with existing animals.
Low down on each side of the vertebra, at the junction of its body with the neural arch, is a large ovate foramen, transversely elongated, and often a little impressed at the border, which is the entrance of the air cell into the bone. These foramina are often one-third of the length of the neck vertebræ in specimens from the Cambridge Greensand, where the neck bones vary from three-quarters of an inch to about two and a half inches in length, and in extreme forms are as wide as long. The width of the interspace between the foramina is one-half the width of the vertebræ, though this character varies with different genera and species. Several species from the Solenhofen Slate have the neck long and slender, on the type of the Flamingo. In others the neck is thick and short—in the Scaphognathus crassirostris and Pterodactylus spectabilis. Some genera with slender necks have the bones preserved with a curved contour, such as might suggest a neck carried like that of a Llama or a Camel. The neck is occasionally preserved in a curve like a capital S, as though about to be darted forward like that of a bird in the act of striking its prey. The genera of Pterodactyles with short necks may have had as great mobility of neck as is found among birds named Ducks and Divers; but those Pterodactyles with stout necks, such as Dimorphodon and Ornithocheirus, in which the vertebræ are large, appear to have been built more for strength than activity, and the neck bones have been chiefly concerned in the muscular effort to use the fighting power of the jaws in the best way.
FIG. 25. CERVICAL VERTEBRA OF ORNITHOCHEIRUS
From the Cambridge Greensand
THE BACK
The region of the back in a Pterodactyle is short as compared with the neck, and relatively is never longer than the corresponding region in a bird. The shortness results partly from the short length of the vertebræ, each of which is about as long as wide. There is also a moderate number of bones in the back. In most skeletons from Solenhofen these vertebræ between the neck and girdle of hip bones number from twelve to sixteen. They have a general resemblance in form to the dorsal vertebræ in birds. The greatest number of such vertebræ in birds is eleven. The number is small because some of the later vertebræ in birds are overlapped by the bones of the hip girdle, which extend forward and cover them at the sides, so that they become blended with the sacrum. This region of the skeleton in the Dimorphodon from the Lias is remarkable for the length of the median process, named the neural spine, which is prolonged upward like the spines of the early dorsal vertebræ of Horses, Deer, and other mammals. In this character they differ from living reptiles, and parallel some Dinosaurs from the Weald. The bones of the back in Ornithocheirus from the Cambridge Greensand show the under side to be well rounded, so that the articular surfaces between the vertebræ, though still rather wider than deep, are much less depressed than in the region of the neck. The neural canal for the spinal cord has become larger and higher, and the sides of the bone are somewhat compressed. Strong transverse processes for the support of the ribs are elevated above the level of the neural canal, at the sides of vertebræ compressed on the under sides, and directed outward. Between these lateral horizontal platforms is the compressed median neural spine, which varies in vertical height. The articulation of the ribs is not seen clearly. Isolated ribs from the Stonesfield Slate have double-headed dorsal ribs, like those of birds. In some specimens from the Solenhofen Slate like the Scaphognathus, in the University Museum at Bonn, dorsal ribs appear to be attached by a notch in the transverse process of the dorsal vertebra, which resembles the condition in Crocodiles. Variations in the mode of attachment of ribs among mammals may show that character to be of subordinate importance. Von Meyer has described the first pair of ribs as frequently larger than the others, and there appear in Rhamphorhynchus to be examples preserved of the sternal ribs, which connect the dorsal ribs with the sternum. Six pairs have been counted. A more interesting feature in the ribs consists in the presence behind the sternum, which is shorter than the corresponding bone in most birds, of median sternal ribs. They are slender V-shaped bones in the middle line of the abdomen, which overlapped the ends of the dorsal ribs like the similar sternal bones of reptiles. Such structures are unknown among Birds and Mammals. There is no trace in the dorsal ribs of the claw-like process, which extends laterally from rib to rib as a marked feature in many birds. Its presence or absence may not be important, because it is represented by fibro-cartilage in the ribs of crocodiles, and may be a small cartilage near the head of the rib in serpents, and is only ossified in some ribs of the New Zealand reptile Hatteria. So that it might have been present in a fossil animal without being ossified and preserved. Although the structure is associated with birds, it is possibly also represented by the great bony plates which cover the ribs in Chelonians, and combine to form the shield which covers the turtle's back. The structure is as characteristic of reptiles as of birds, but is not necessarily associated with either.