Hermann von Meyer, the historian of the Ornithosaurs of the Lithographic Slate, as early as 1837 described some Pterodactyle bones from the Lias of Franconia, which showed that air was admitted into the interior of the bones by apertures near their extremities, which, from this circumstance, are known as pneumatic foramina. He drew the inference, naturally enough, that such a structure is absolute proof that the Pterodactyle was a flying animal. It was not quite the right form in which the conclusion should have been stated, because the Ostrich and other birds which do not fly have the principal bones pneumatic. Afterwards, in 1859, the larger bones which Professor Sedgwick, of Cambridge, transmitted to Sir Richard Owen established this condition as characteristic of the Flying Reptiles of the Cambridge Greensand. It was thus found as a distinctive structure of the bones both at the beginning and the close of the geological history of these animals. Von Meyer remarks that the supposition readily follows that in the respiratory process there was some similarity between Pterodactyles and Birds. This cautious statement may perhaps be due to the circumstance that in many animals air cavities are developed in the skull without being connected with organs of respiration. It is well known that the bulk of the Elephant's head is due to the brain cavity being protected with an envelope formed of large air cells. Small air cells are seen in the skulls of oxen, pigs, and many other mammals, as well as in the human forehead. The head of a bird like the Owl owes something of its imposing appearance to the way in which its mass is enlarged by the dense covering of air cells in the bones above the brain, like that seen in some Cretaceous Pterodactyles. Nor are the skulls of Crocodiles or Tortoises exceptions to the general rule that an animal's head bones may be pneumatic without implying a pneumatic prolongation of air from the lungs. The mere presence of air cells without specification of the region of the skeleton in which they occur is not remarkable. The holes by which air enters the bones are usually much larger in Pterodactyles than in Birds, but the entrance to the air cell prolonged into the bones is the same in form and position in both groups. So far as can be judged by this character, there is no difference between them. The importance of the comparison can only be appreciated by examining the bones side by side. In the upper arm bone of a bird, on what is known as the ulnar border, near to the shoulder joint, and on the side nearest to it, is the entrance to the air cell in the humerus. In the Pterodactyle the corresponding foramen has the same position, form, and size, and is not one large hole, but a reticulation of small perforations, one beyond another, exactly such as are seen in the entrance to the air cell in the bone of a bird, in which the pneumatic character is found. For it is not