There is no organ more distinctive between hot-blooded animals on the one hand, and cold-blooded animals on the other, than the brain. In the cold-blooded groups, or those in which respiration is feeble and circulation imperfect, that is to say, in existing fishes, amphibians, and reptiles, the parts of the brain are arranged one behind another, so that when looked upon from above, a portion called the optic lobes intervenes between the anterior masses called the cerebrum and the posterior mass called the cerebellum. In the hot-blooded groups, or those with an enormous extent of lung-surface for oxidation of the blood and a four-celled heart for its rapid circulation, that is to say, in birds and mammals, the front part of the brain called the cerebrum is immensely developed in proportion to the other parts, and abuts against the cerebellum and more or less completely covers the optic lobes, which in birds are squeezed out to the sides. The Pterodactyle brain is of this latter kind. And it being taken as a postulate that this kind of brain is the product of the organization which produces hot blood, it follows that the Pterodactyle was a hot-blooded animal. Again, the Pterodactyle has perforations for pneumatic cells in many of the bones. There is no structure in the animal kingdom more distinctive of a Class of animals than air-cells perforating the limb-bones. They are connected with a peculiar kind of lung and heart--those of the bird; for in this Class the bronchial tubes open on the outer surface of the lungs into air-cells, which are prolonged through the body into the bones. They follow the blood-vessels, and are most developed in the part of the body most used. In some lizards, as the Chameleon, the sack-like lung at its distal termination is as simple as the air-cells of a bird; but those air-cells are not comparable with the bird's air-cells, since they are not prolongations of the bronchial tubes through the walls of the lungs. And it cannot be inferred that a reptile with wings would develop air-cells like those of a bird: in the first place, because those mammals which have wings do not develop air-cells; and, in the second place, because there is nothing in existing nature to lead any one to think that reptiles might have wings. The mammalian lung is better comparable to that of a bird than is the Chameleon lung, and therefore the air-cell structure might with better reason have been anticipated to occur in the Chiroptera than in a Lizard-ally, if it were dependent on the development of wings. Moreover, among Struthious birds the legs have more of the air-cell prolongations than the wings. Therefore, being a peculiar Avian structure which only exists in association with the Avian heart and lung, it follows that because the Pterodactyle had the pneumatic foramina it also had the structures of which they are the evidence, viz. lung and heart formed on the bird plan.