in Natural History, a well-known part of fishes, consisting of a membrane supported by rays, or little bony or cartilaginous osicles.

The office of the fins has commonly been supposed to be analogous to that of feathers in fowls; and to assist the fish in its progressive motion, or swimming; but the later naturalists find this a mistake.

The tail is the great instrument of swimming; the fins only serve to keep the fish upright, and prevent vacillation or wavering. See Ichthyology Index.