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1. Incorporation of calcium and carbonate into shell has been studied in the scallop Argopecten irradians using 45 Ca and 14 C-bicarbonate. 2. The incorporation of 45 Ca and 14 C-carbonate into shell was linear with time after a lag period of 1 to 2 hours. The shell-forming mantle tissue attained a steady state with respect to 45 Ca in the sea water medium within 2 hours. 3. The molar ratio of 45 Ca to 14 C-carbonate deposited in shell was not significantly different from unity during 5 hours. 4. The rate of incorporation of 14 C-carbonate into shell was highest at the ventral edge and extremely low in the central and hinge areas. 5. The rate of incorporation at the ventral shell edge did not change with increase in shell size. 6. The rate of incorporation of carbonate was low at night when growth ridges form and increased 3-fold at midday when growth ridges are not being formed. 7. The protein content of the shell ridges was 32.9 ± 3.9% and the protein content of the shell including ridges was 16.0 ± 2.1%.

Morphologic studies of single crystals of aragonite within Codiacean algae reveal characteristic crystal forms produced by two distinctly different modes of calcification. Diagnostic serrated crystals (1 micrometer in length) of aragonite originating within the extracellular sheaths of capitular filaments are incorporated into modern lime sediments and may serve as effective tracers for particles of algal origin. Intracellular calcification within Penicillus dumetosus, previously unueported, is represented by doubly terminated aragonite crystals ranging in size from 48 to 160 micrometers.