annulus

(ănˈyə-ləs)

1. A ringlike figure, part, structure, or marking, such as a growth ring on the scale of a fish.
2. a. A ring or group of thick-walled cells around the sporangia of many ferns that functions in spore release.
   b. The ringlike remains of a broken partial veil, found around the stipes of certain mushrooms.
3. Mathematics The figure bounded by and containing the area between two concentric circles.