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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.

Extremely small shells, less than 2 mm. in length, smooth, except for a faint cord or spiral thread on the periphery. Nucleus large, of $1 1/2$ 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:

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).