Strophitus undulatus (Say, 1817)
Strophitus undulatus, INHS 1509. North Fork Vermilion River, Vermilion County, Illinois.
Other common names Strange floater, sloughfoot, creeper.
Key characters Elliptical to oval shell, thin to relatively thick, green, dark brown, or black, with pronounced beak sculpture and poorly developed hinge teeth. Nacre usually salmon to orange.
Description Shell elliptical, moderately compressed, and thin when young, becoming somewhat inflated and thicker in adults. Anterior end rounded, posterior end bluntly pointed, occasionally truncated. Ventral margin straight to slightly curved. Umbos slightly elevated above the hinge line and located at least one-third from the anterior end. Beak sculpture of two or three pronounced V-shaped ridges. Shell smooth and shiny. Periostracum green with rays in juveniles, becoming chestnut, dark brown, and black in older individuals. Length to 4 inches (10.2 cm).
Pseudocardinal and lateral teeth weakly developed and present only as thickened ridges. Hinge line curved past the umbo. Beak cavity moderately shallow. Nacre salmon or cream-colored, bluish white along the outer margin of the shell.
Habitat Small to medium-sized streams and occasionally large rivers in mud, sand, or gravel.
Status Widespread and common. Threatened in Iowa.