of direction, then into fan-shaped and later paddle-shaped organs of locomotion. Finally, from the jointed paddle, which Gegenbaur has called the archipterygium, there has developed, on the one hand, the rayed pectoral and ventral fins of ordinary fishes, and on the other, in land-creeping animals, jointed legs and arms. As to this the evidence of paleontology is conflicting. An early shark of the Devonian, Cladoselache,
has fan-shaped paired fins so formed and placed. Another shark almost as old, Pleuracanthus, of the Carboniferous, has fins which fit best a totally different theory of origin. Its jointed or archipterygial fin has no resemblance to a fold of skin, but accords better with Gegenbaur's theory that the pectoral limb was at first a modified septum or gill arch. Sharks still older than either (Heterodontidæ) in the Silurian, so far as we can judge by their teeth, are closely related to forms bearing the more specialized type of fin found in the typical
sharks of to-day. Evidently none of these three, as seen in the rocks, represents the real beginning of paired fins in the life of the past. As we shall see, the evidence of comparative anatomy may be consistent with either of these theories, while that of ontogeny or embryology is apparently inconclusive, and that of paleontology seems contradictory.
Development of the Paired Fins in the Embryo.
According to Dr. John A. Ryder ('Embryography of Osseous Fishes,' 1882) "the paired fins in Teleostei, like the limbs of the higher
