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136 American Seashells
Subgenus Onoba H. and A. Adams 1854
Cingula aculeus Gould Pointed Cingula
Nova Scotia to Maryland.
Extremely small, 2.5 mm. in length, elongate, about 5 whorls, no um-
bilicus. Whorls rounded. Suture well-impressed. Aperture ovate wdth a
slightly flaring hp. Color light- to rusty-brown. Spiral sculpture of numer-
ous, microscopic incised Hnes. 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 i % 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
Periphery with thread or cord:
/^^WTZ^^wi- Carpenter (San Pedro south).
Periphery angulate w^/z/n/^ Carpenter (San Pedro south).
Periphery rounded
Genus Rissoina Orbigny 1840
tenuis ^2.nsc\\ (Monterey south).
Shells small, usually less than % 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, pauci-
spiral, 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 pi. 22u).
Key to the Pacific Coast Rissoina
A. Color pure-wliite or bluish white B
Color yellow to light-red; 6 mm., Redondo Beach south
kelseyi Dall and Bartsch
B. Axial ribs strong, less than 20 on the last wliorl C
Axial ribs weak, numerous D