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a class of extinct animals conventionally assigned to the phylum Mollusca. (Some scientists classify the animals as an independent phylum.) The Hyolitha lived from the beginning of the Cambrian to the Permian. The conical or pyramidal shell ranged from 0.1 to 15 cm in length and was sealed at the narrow end. The stoma of the shell closed with an operculum. In most species the dorsal and ventral sides of the shell were highly sculptured. In many the shell cavity was divided by transverse septa into a number of air chambers and one dwelling chamber. Some scientists believe that the shell was bivalved, that is, it consisted of conical and flat valves. The wall of the shell is calcareous and two-layered. Some species had a pair of long slender appendages departing from the stoma. The Hyolitha lived in the sea. They are very important in the stratigraphy of Cambrian deposits.