The apparatus of flight in the “Pterosaurs” is of the most remarkable character, and most resembles the “wing” of a Bat, though very different in some important particulars.  The “wing” of the Pterosaurs is like that of Bats, namely, in consisting of a thin leathery expansion of the skin which is attached to the sides of the body, and stretches between the fore and hind limbs, being mainly supported by an enormous elongation of certain of the digits of the hand.  In the Bats, it is the four outer fingers which are thus lengthened out; but in the Pterosaurs, the wing-membrane is borne by a single immensely-extended finger (fig. 178).  No trace of the actual wing-membrane itself has, of course, been found fossilised; but we could determine that the “Pterodactyles” possessed the power of flight, quite apart from the extraordinary conformation of the hand.  The proofs of this are to be found partly in the fact that the breast-bone was furnished with an elevated ridge or keel, serving for the attachment of the great muscles of flight, and still more in the fact that the bones were hollow and were filled with air—­a peculiarity wholly confined amongst living animals to Birds only.  The skull of the Pterosaurs is long, light, and singularly bird-like in appearance—­a resemblance which is further increased by the comparative length of the neck and the size of the vertebrae of this region (fig. 178).  The jaws, however, unlike those of any existing Bird, were, with one exception to be noticed hereafter, furnished with conical teeth sunk in distinct sockets; and there was always a longer or shorter tail composed of distinct vertebrae; whereas in all existing Birds the tail is abbreviated, and the terminal vertebrae are amalgamated to form a single bone, which generally supports the great feathers of the tail.

Modern naturalists have been pretty generally agreed that the Pterosaurs should be regarded as a peculiar group of the Reptiles; though they have been and are still regarded by high authorities, like Professor Seeley, as being really referable to the Birds, or as forming a class by themselves.  The chief points which separate them from Birds, as a class, are the character of the apparatus of flight, the entirely different structure of the fore-limb, the absence of feathers, the composition of the tail out of distinct vertebrae, and the general presence of conical teeth sunk in distinct sockets in the jaws.  The gap between the Pterosaurs and the Birds has, however, been greatly lessened of late by the discovery of fossil animals (Ichthyornis and Hesperornis) with the skeleton proper to Birds combined with the presence of teeth in the jaws, and by the still more recent discovery of other fossil animals (Pteranodon) with a Pterosaurian skeleton, but without teeth; whilst the undoubtedly feathered Archoeopteryx possessed a long tail composed of separate vertebrae.  Upon the whole, therefore, the relationships of the Pterosaurs cannot be regarded as absolutely settled.  It seems certain, however, that they did not possess feathers—­this implying that they were cold-blooded animals; and their affinities with Reptiles in this, as in other characters, are too strong to be overlooked.