Thus the heart and lungs are exactly such as would have been inferred from the brain, and, like it, they are avian. And so important are these vital structures all taken together, that the inference from them upon an animal's affinities would overbear all other evidence that could be adduced except reproduction; for they demonstrate the plan on which an animal was built, and are the motor power which enabled it to use its skeleton in a way that stamped upon it a peculiar form. In the head such structures as are preserved conform with slight variations to the avian plan. Other Ornithosaurians show in the parts which are not preserved in Cambridge specimens some characters which are not avian; they are in part as much mammalian as reptilian, and in a few points entirely reptilian. But it might be misleading to take German specimens into consideration in forming an estimate of the Pterodactyles of the Cambridge Greensand, which were probably a different ordinal group, and may have had material differences in structure. The vertebral column as a whole is distinctive. The neck and sacrum are mammalian, and the tail reptilian. The procœlous vertebræ are characteristic of reptiles, but in some animals, as Chelonians, they vary in different regions of the body; and among amphibians the character is inconstant in genera nearly allied. The hind-limb is in part mammalian and in part avian; if there be any reptilian characters in the foot, they are not less mammalian. The os innominatum is avian and mammalian. The pectoral girdle is avian. The fore-limb is avian and mammalian. The wing-finger is distinctive, though formed on the avian plan. Thus, if with an avian basis some parts of the skeleton present points of agreement with reptiles, in other points there are resemblances with mammals not less characteristic. These phænomena do not show that in so far the animal is a mammal or a reptile, but only that mammals, ornithosaurians, and reptiles have had a common origin, and that while they have been differentiated so as to form separate classes they have severally retained characters which formerly were united in one class. It is a skeleton intermediate between reptiles and mammals, and well distinguished by mammalian, reptilian, and peculiar characters, from birds. It therefore forms a parallel group with birds, displaying the ornithic organization in a differently modified skeleton. Yet it differs more from existing birds than they differ among themselves, for the discrepancies are in points of structure in which all existing birds agree: they are in having teeth, in the procœlous centrum, in the separate condition of the carpal and metacarpal (and of the tarsal and metatarsal) bones; in having more than two bones in the fore-arm, in the sacrum formed of few vertebræ, in the expanded pubic (and prepubic) bones, in a long neck to the femur, and in the modification of the wing by the great development of the phalanges of one finger.