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the only extant genus of marine mollusks of the class Monoplacophora. The body is covered with a thin miterlike shell, whose apex is near the anterior end; the shell is up to 37 mm long. There are five or six pairs of pectinate gills in the mantle groove and eight pairs of body-retractor muscles. The pharynx has a radula, and the heart has two ventricles and four auricles. There are six pairs of kidneys and two pairs of gonads. The sexes are separate, and fertilization is external. The neopilina feed on detritus. There are six known species, found at depths of 1,800 to 6,500 m in the Pacific, Indian, and southwestern Atlantic oceans.