Musculium, Pisidium, and Sphaerium
(Family Sphaeriidae)
Other common names: Pillclams, nutclams.
Key characters: Small, whitish or cream-colored shell with numerous, very fine concentric rows of elevated lines (striae) covering the surface. The right valve has one cardinal tooth and two pairs of lateral teeth. The left valve has two cardinal teeth and two lateral teeth.
Similar species: Corbicula species.
Description: Shell rounded to slightly oval and inflated. Anterior and posterior margins rounded. Umbos centrally located and slightly elevated above the hinge line. Periostracum whitish or cream-colored, with numerous, very fine concentric rows of elevated lines (striae) covering the surface of the shell. The posterior end of the shell is longer in Sphaerium and Musculium, and just the reverse in Pisidium. Length to 0.5 inch (1.3 cm).
The right valve has one cardinal tooth and two pairs of lateral teeth. The left valve has two cardinal teeth and two lateral teeth. Beak cavity moderately deep. Nacre white.
Habitat: Lakes and streams of all sizes in silt, mud, sand, or gravel.
Status: Little is known regarding the status of fingernailclams and peaclams. They have disappeared from many streams where they were formerly abundant (such as the Illinois River).