Villosa iris (Lea, 1829)
Other common names: Rainbow shell.
Key characters: Small, relatively thin, and elongate shell, distinct double-looped beak sculpture, yellow, with numerous broken green rays.
Similar species: Ellipse, rayed bean, fatmucket, mucket.
Description: Shell small, elongate, relatively thin, and compressed (males) to moderately inflated (females). Anterior end rounded, posterior end rounded (females) to bluntly pointed (males). Umbos even with or slightly elevated above hinge line. Beak sculpture of four to six distinct, double-looped bars. Periostracum yellow or greenish yellow, with dark green rays, often interrupted. Length to 3 inches (7.6 cm).
Pseudocardinal teeth small, triangular, and somewhat divergent; two in the left valve, one in the right. Lateral teeth long, thin, and straight to slightly curved. Beak cavity shallow. Nacre silvery white and highly iridescent on the posterior half, giving this species its common name.
Habitat: Small to medium streams in coarse sand or gravel.
Status: Widespread but disappearing from the western part of its range. Endangered in Illinois and Wisconsin.