Subgenus Onoba H. and A. Adams 1854
Nova Scotia to Maryland.
Extremely small, 2.5 mm. in length, elongate, about 5 whorls, no umbilicus. Whorls rounded. Suture well-impressed. Aperture ovate with a slightly flaring hp. Color light- to rusty-brown. Spiral sculpture of numerous, microscopic incised lines. Below the suture there are numerous, short, axial riblets. Common in shallow water.
Genus Amphithalamus Carpenter 1865
Extremely small shells, less than 2 mm. in length, smooth, except for a faint cord or spiral thread on the periphery. Nucleus large, of 11⁄2 whorls which are finely pitted like a thimble. The most striking character is a thin bridge separating the inner lip from the open umbilicus. There are 3 species in southern California:
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Periphery with thread or cord: | |||
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Genus Rissoina Orbigny 1840
Shells small, usually less than 1⁄8 inch in length, Generally white in color, with strong or weak axial ribs, occasionally with fine spiral, incised lines. Aperture semilunar and somewhat flaring. Operculum corneous, thick, paucispiral, with a claviform process on the inner surface. We have presented nearly all of the species known to both sides of the United States in the form of a key (see pl. 22u).
Key to the Pacific Coast Rissoina
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