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In page Endocerida:

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Endocerids may have had a relatively small body chamber as well as a proportionally large siphuncle that in some reached nearly half the shell diameter, suggesting much of the visceral mass may have been housed within the siphuncle itself rather than just in the body chamber as with other nautiloids (Teichert, 1964). Endocerids are primarily distinguished by the presence of calcareous deposits, known as endocones, formed in the more apical portion of the siphuncle and thought to counterweight the animal’s body. The chambers (camerae) of endocerids are always free of organic deposits, unlike other orders such as the Michelinocerida and Actinocerida.