254 HISTORICAL PALEONTOLOGY. 



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 struc- 

 ture 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 un- 

 doubtedly feathered Archaopteryx possessed a long tail com- 

 posed 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. 



The Pterosaurs are wholly Mesozoic, ranging from the Lias 

 to the Chalk inclusive; and the fine-grained Lithographic Slate 

 of Solenhofen has proved to be singularly rich in their remains. 

 The genus Pterodactylus itself has the jaws toothed to the ex- 

 tremities with equal-sized conical teeth, and its species range 

 from the Middle Oolites to the Cretaceous series, in connec- 

 tion with which they will be again noticed, together with the 

 toothless genus Pteranodon. The genus Dimorphodon is Li- 



