coccolith

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several minute calcite plates that make up the external covering of certain haptophyte phytoplankton and in a fossilized state form chalk and limestone deposits.

from The Century Dictionary.

• noun 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.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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