Fin
2 of the 7 encyclopedias on this shelf carry an entry for Fin. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the organ by which locomotion is effected in a fish. As a rule fins consist of a membrane supported by rays. Of these organs the two pectoral fins, so called from being situated on the breast, where they are just behind the branchial aperture, are modifications of the anterior limbs in other vertebrata. The ventral fins, so called from being, as a rule, situated on the belly, correspond to the hind limbs in other vertebrata. Often there are also one or more dorsal fins on the back, two anal fins near the anus, while the tail is technically called the caudal fin. In carpentry, a tongue on the edge of a board. In commerce, a blade of whalebone. In machinery, a slip inserted longitudinally into a shaft or arbor, and left projecting so as to form a guide for an object which may slip thereon, but not rotate; a spline or feather. In molding, a mark or ridge left in casting at the junction of the parts of the mold.
The Encyclopedia of Founding (1892)
Metal that flows past the casting in a thin ridge at the joining edges of a mould. In some cases the edges are purposely pared down to prevent any possibility of the two edges meeting. Especially is this rule to be observed in loam and dry-sand work, where in the event of too close contact the mould is damaged by a crushed joint. The term crush is common when any part of a mould is damaged by undue pressure, etc.