According to the late Professor Marsh, the limbs of Coelurus and its allies, which at present are imperfectly known, are in some cases pneumatic. Therefore there is a closer fundamental resemblance between some carnivorous Dinosaurs and Pterodactyles than might have been anticipated. But the skull of Coelurus is unknown, and the fragments of the skeleton hitherto published are insufficient to do more than show that the two types were near in kindred, though distinct in habit. Each has elaborated a skeleton which owes much to the common stock which transmitted the vital organs, and the tendency of the bones to take special forms; but which also owes more than can be accurately measured to the action of muscles in shaping the bones and the influence of the mechanical conditions of daily life upon the growth of the bones in both of these orders of animals. Enough is known to prove that all Dinosaurs cannot be regarded as Ornithosaurs which have not acquired the power of flight; though the evidence would lead us to believe that the primitive Ornithosaur was a four-footed animal, before the wing finger became developed in the fore limb as a means of extending a patagial membrane, like the membrane which in the hind limb of Dimorphodon has bent the outermost digit of the foot upward and outward to support the corresponding organ of flight extending down the hind legs. It may thus be seen that the characters of Ornithosaurs which have already been spoken of as Reptilian, as distinguished from the resemblances to Birds, may now with more accuracy be regarded as Dinosaurian. The Dinosaurs, like Pterodactyles, must be regarded as intermediate in some respects between Reptiles and Birds. The resemblances enumerated would alone constitute a partial transition from the Reptile to the Bird, although no Dinosaurs have organs of flight; many are heavily armoured with plates of bone, and few, if any, approximate in the technical parts of the skeleton to the Bird class, except in the hind limbs. Yet Dinosaurs have sometimes been regarded as standing to Birds in the relation of ancestors, or as parallel to an ancestral stock.