1. The structure and development of the tails of Pleuronectes platessa L. and of Gadus morrhua L. are described, with some reference to Gadus merlangus L.
2. In Pleuronectes: (a) seven hypural cartilages are identified, the most posterior becoming a dorsal element in the definitive tail while the next three fuse to form the upper of the two hypural bones borne by the terminal vertebra; (b) the penultimate vertebra comes to bear two dorsal and two ventral arches, probably basalia, as a result of their fusion during development; (c) the last neural and haemal arch bear traces of transient interbasalia at their first appearance.
3. In Gadus: (a) the tail resembles that of Pleuronectes in certain respects, including the general arrangement of the dorsal and ventral skeletal elements, the appearance of transient interbasalia in connexion with the last neural and haemal arch, and the fusion of two dorsal and ventral arches, although these come to be borne by the ante-penultimate vertebra instead of by the penultimate vertebra; (b) the embryonic axial lobe of the fin is median (instead of dorsal as it is in Pleuro-nectes) as a result of the upper half of the definitive fin being formed by the backgrowth of a dorsal fin-fold.
4. The two types of fin are compared, and it is concluded that the Gadoid fin could have been derived by the fusion of a homocercal fin, of a type more primitive than that of Pleuro-nectes, with a dorsal and ventral fin-fold, accompanied by the loss of the most terminal of the hypurals characteristic of the normal homocercal fin.