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. THE BACK