Page:American Seashells (1954).djvu/186

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 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:

Periphery without spiral line

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lacunatus Carpenter (San Pedro south).

Periphery with thread or cord:

Periphery angulate

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inclusus Carpenter (San Pedro south).

Periphery rounded

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tenuis Bartsch (Monterey south).

 

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

A. Color pure-white or bluish white

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B

Color yellow to light-red; 6 mm., Redondo Beach south

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kelseyi Dall and Bartsch

B. Axial ribs strong, less than 20 on the last whorl

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C

Axial ribs weak, numerous

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D