2. (Arch.) The keystone of an arch. [R.] --Gwilt.
3. (Geom.) The distance from a point in a curve to the chord;
also, the versed sine of an arc; -- so called from its
resemblance to an arrow resting on the bow and string.
[Obs.]
4. (Anat.) The larger of the two otoliths, or ear bones,
found in most fishes.
5. (Zo["o]l.) A genus of transparent, free-swimming marine
worms having lateral and caudal fins, and capable of
swimming rapidly. It is the type of the class
Ch[ae]tognatha.