THE SPIRE SHELLS AND - FLOOD SHELLS - FAMILY RISSOIDAE Shell small, top-shaped or elongated; mouth rounded, scarcely channeled. A family of small marine mollusks living on seaweeds, to which they are suspended by a mucous thread.
Genus RISSOA, Freim.
Characters of the family. Two hundred species, abundant chiefly in shallow seas.
R. minuta, Totten, is a smooth, blunt-pointed shell, with rounded, finely striated whorls. It is yellowish brown and clings by a thread to the leaves of seaweeds. Length, inch.
Habitat.— New England to New Jersey.
R. acutilirata Cpr., to -I inch long, is worthy of ex amination under a lens. Its brownish yellow surface is marked by sharp intersecting ribs. 1 t is found on seaweed.
Habitat.— San Diego, Cal.
Genus RISSOINA, D'orb.
Resembles Rissoa except that the aperture and operculum are ear-shaped, and the latter has a blade-like projection on its inner surface. Warm coasts.
Genus BARLEEIA, Clark Shell smooth, spiral elongated; aperture round; operculum not spiral; with internal projection.
Genus AMNICOLA, Gld. and Hald.
Shell small, short, ovate or globose, thin, smooth; apex not acute; aperture broadly ovate, not oblique; lip thin, sharp; not projecting below; operculum horny, spiral. Foot short, broad, rounded behind; tentacles blunt; snout short; egg cases pod-like, short, one egg in each. A fresh-water snail of North
America. Species few.
Habitat.—Hudson Bay region to Wisconsin and Virginia.
Shell obliquely ovate, thick, smooth, spire blunt; inner lip flat and callous, outer lip spreading, especially below; operculum horny. Egg cases lens-shaped, containing many eggs. Found in fresh water in Oregon and California.
A green species, F. virens, Lea, somewhat slender and oblique, but very thick, and a very stout, almost spherical one. F. fusca, Hald., reddish, with white lip, and the size of a pea, inhabits Oregon streams.
Genus POMATIOPSIS, Tryon.
Shell small, thin, elongated, of few very much rounded whorls; aperture round, lip continuous, extended or reflected; 191 The Spire Shells and Flood Shells operculum horny. Foot very broad; snout extensible; gill present, but the mollusk breathes air.
Michigan and Missouri to Georgia and New York. P. Californica, PiIs., has a turreted-conic, thin, brown shell with a pit underneath the rounded whorls. Length, i inch. Small streams about San Francisco Bay.