[CHAPTER X.]
FOSSIL SHELL-FISH (MOLLUSCA).
Molluscan Characters.—
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The phylum or sub-kingdom Mollusca is a group of soft-bodied animals (mollis, soft), which, although having no external skeleton, usually possess the protective covering of a shell. This shell is secreted from the outer skin or mantle, and is composed of carbonate of lime (calcareous) with a varying proportion of organic material.
Hard Parts.—
Fossil molluscan remains consist practically of the shells, but the calcareous apertural lid (operculum) of some kinds is often preserved, as in Turbo and Hyolithes; or the horny lids of others, as Bithynia of the European Pleistocene “brick earths.” The cuttle-fishes have hard, horny beaks and internal bones, and the latter are frequently found fossil in Australia.
Characters of Pelecypoda.—