Definitions of shallow—water marine climatic zones that equate the northern limit of climatic zones with minimum temperatures are shown to be not critical. The critical factor that probably determines the limits of marine shallow—water climates is the number of consecutive days or months that shallow sea water is at temperatures required for reproduction and early growth. The relations that appear to exist between duration of marine water temperature or climate and molluscan provinces are: (1) Inner tropical includes molluscan species that live in water that is 20 C or nearly 20 C for 6 or more months, and with no months cooler than 18 C. (2) Outer tropical includes those species that live in water 20 C. For only 4 months, in water that is 18 C for approximately 6 months, and for no months cooler than 10 C. (3) Warm temperate includes species that live in water approximately 15 or 18 C for almost 4 continuous and in water no colder than 10 C. (4) Mild temperate contains species living in marine water that is near or slightly below 15 C. for 3 or 4 months, and cooler than 10 C for nearly 6 months. (5) Cool temperate includes species that live in water that is 10 C for only 3 or 4 months of the year. (6) The Southern boundary of cold climate is near the southern limit of winter ice, and the distribution of cold water molluscan species may result from the distribution of polar ice.

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