backward in time to the Upper Secondary rocks, as seen in the Archæopteryx. The Avian characters of Pterodactyles are the predominant parts of their organisation, for the conditions of the brain and lungs shown by the moulds of the brain case and the thin hollow bones with conspicuous pneumatic foramina, give evidence of a community of vital structures with Birds, which is supported by characters of the skeleton. If any classificational value can be associated with the distribution of the pneumatic foramina as tending to establish membership of the same class for animals fashioned on the same plan of soft organs, the evidence is not weakened when a community of structures is found to extend among the bones to such distinctive parts of the skeleton as the sternum, shoulder-girdle, bones of the fore-arm and fore-leg; for in all these regions the Pterodactyle bones are practically indistinguishable from those of Birds. This is the more remarkable because other parts of the skeleton, such as the humerus and pelvis, show a partial resemblance to Birds, while the parts which are least Avian, like the neck bones, have no tendency to vary the number of the vertebræ, in the way which is common among Birds, following more closely the formula of the seven cervical vertebræ of Mammals. It would therefore appear from the vital community of structures with Birds, that Pterodactyles and Birds are two parallel groups, which may be regarded as ancient divergent forks of the same branch of animal life, which became distinguished from each other by acquiring the different condition of the skin, and the structures which were developed in consequence of the bony skeleton ministering to flight in different ways; and with different habit of terrestrial progression, this extinct group of animals acquired some modifications of the skeleton which Birds have not shown. There is nothing to suggest that Pterodactyles are a branch from Birds, but their relation to Birds is much closer, so far as the skeleton goes, than is their relation with the flightless Dinosaurs, with which Birds and Pterodactyles have many characters in common. On the theory of elimination of character which I have used to account for the disappearance of some Mammalian characters from the Pterodactyle, that loss is seen chiefly in the removal of the parts which have left a Reptilian articulation of the lower jaw with the skull, and the articulation of the vertebræ throughout the vertebral column by a modified cup-and-ball form of joint. The furculum of the Bird is always absent from the Pterodactyle. No specimen has shown recognisable clavicles or collar-bones. Judged by the standard of existing life, Pterodactyles belong to the same group as Birds, on the evidence of brain and lungs, but they belong to a different group on account of the dissimilar modifications of the skeleton and apparent absence of feathers from the skin.