coccolith

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A microscopic calcite skeletal plate that protects certain marine phytoplankton and in a fossilized state forms chalk and limestone deposits.

Century Dictionary and Cyclopedia

1. n. A minute round organic body, consisting of several concreted layers surrounding a clear center, found in profusion at great depths in the North Atlantic ocean embedded in matter resembling sarcode. It is probable that the coccoliths are unicellular algæ.
2. n. Harting has found that minute calcareous disks are separated out of a solution of lime sulphate or lime chlorid by the action of ammonia generated by the decomposition of organic matter, and therefore it has been inferred that the coccoliths may be separated from the sea-water whenever organic decomposition is in progress in the presence of lime sulphate.

Wiktionary

1. n. biology A microscopic skeletal plate of calcite on the surface of certain marine phytoplankton; it forms chalk and limestone when fossilized

GNU Webster's 1913

1. n. (Biol.) One of a kind of minute, calcareous bodies, probably vegetable, often abundant in deep-sea mud.