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 vertebræ 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 vertebræ; whereas in all existing Birds the tail is abbreviated, and the terminal vertebræ 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 vertebræ, 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 vertebræ. 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. [Illustration: Fig. 179--_Rhamphorhynchus Bucklandi_, restored. Bath Oolite, England. (After the late Professor Phillips.)]