These illustrations may be accepted as demonstrating a relationship between the Ornithosaurs and Dinosaurs now compared, which can only be explained as results of influence of a common parentage upon the forms of the bones. But more interesting than resemblances of that kind is the similarity that may be traced in the way in which air is introduced into cavities in the bones in both groups. In some of the imperfectly known Dinosaurs, like Aristosuchus, Coelurus, and Thecospondylus, the bone texture is as thin as in Pterodactyles, and the vertebræ are excavated by pneumatic cavities, which are amazing in size when compared with the corresponding structures in birds, for the vertebra is often hollowed out so that nothing remains but a thin external film like paper for its thickness. In the Dinosaurian genus Coelurus this condition is as well marked in the tail and back as it is in the neck. The essential difference from Birds appears to be that in the larger carnivorous Dinosaurs the pneumatic condition of the bones is confined to the vertebral column; while Birds and Pterodactyles have the pneumatic condition more conspicuously developed in the limb bones. The pneumatic skeleton, however, appears to be absent from the herbivorous types like Iguanodon and all Dinosaurs which have the Bird-like form of pelvis, and are most Bird-like in the forms of bones of the hind limb. It is possible that some of the carnivorous Dinosaurs also possessed limb bones with pneumatic cavities. Many of those bones are hollow with very thin walls. If their cavities were connected with the lungs the foramina are inconspicuous and unlike the immense holes seen in the sides of the vertebræ.