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 



/^^WTZ^^wi- Carpenter (San Pedro south). 



Periphery with thread or cord: 



Periphery angulate w^/z/n/^ Carpenter (San Pedro south). 



Periphery rounded 



tenuis ^2.nsc\\ (Monterey south). 



Genus Rissoina Orbigny 1840 



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 



