Electron photomicrographs of replicated surfaces of fossil Coccolithophorids reveal a wealth of minute structural detail. The complex nature of large coccoliths has been suspected from features which can be seen in a light microscope. However, most of the minute structures are visible only in the electron microscope.

Discoliths consist of about 100 radially arranged peripherally branching platelets, each about 1/5 micron wide. Rhabdoliths are extremely complex. The "basal plate" is in reality made up of about 50 wedge-shale platelets arranged in an imbricate fashion. The stem also appears to be made of thin imbricate platelets. Coccolithites gammation possesses a complex circular ridge surrounding the central pore. About nine widely spaced shallow grooves, which may represent sutures, are present on the distal surface of this species. The margin is finely dentate, the individual denticles being only about 1/40 micron across. Broken edges of pentaliths of Braarudosphaera show an apparent laminar structure; however, no traces of fine structure have yet been observed on the surface of the pentaliths. /P>

Discoasterids appear to be much more coarsely constructed than coccoliths, confirming observations made with the light microscope.

Analysis of submicronic structure will be of major importance in establishing a genetic classification of the Coccolithophorids and related nannoplankton.

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