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Bellerophontida
The Bellerophontida is a taxonomic order of extinct marine mollusks that are found in the fossil record from the Lower Cambrian to the Lower Triassic. They are considered by some experts to be primitive sea snails with primarily symmetrically coiled shells, marine gastropod mollusks.
R.C Moore 〔R.C. Moore, 1952, Gastropods, Invertebrate Fossils〕 and J. Brooks Knight ''et al'' 1960 〔Treatise on Invertebrate Paleontology, Part I Mollusca 1〕 recognized bellerophonts as true gastropods, placing them as the suborder Bellerophontina in the sublass Prosobranchia, order Archaeogastropoda. Some more recent workers in the field have expressed uncertainty as to the taxonomic placement of this group.
== Shell morphology==
Shells of bellerophontids are most commonly isotrophic (coiled symmetrically), rarely slightly asymmetric. Most are closely coiled with some general more openly coiled, cyrtiform; predominately with a median slit in the upper lip of the aperture, or with a series of openings known as tremata, usually generating a selenizone which was probably exhalant in function. The shell wall is of variable thickness and no evidence of nacre or of an operculum has been found. Nothing is known of soft part anatomy of the group, although a simple pair of retractor muscles is inferred from muscle scars. Isotrophic symmetry also suggests paired and equal nephridia and ctenidia (kidneys and gills).〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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