Plethobasus cicatricosus (Say, 1829)
Other common names: None.
Key characters: Oblong or elongate shell, light brown, with a row of large tear-shaped tubercles running down the side of the shell.
Description: Shell oblong, elongate, thick, and moderately inflated. Anterior end rounded, posterior end broadly rounded. Dorsal and ventral margins curved. Umbos low, directed forward, and not elevated above the hinge line. Shell smooth, except for a single row of large pustules or knobs on the center of the valve, running from below the umbo to the edge. Periostracum yellow or light brown in juveniles, becoming chestnut to dark brown in adults. Faint green rays present on the umbo. Length to 5 inches (12.7 cm).
Pseudocardinal teeth large and well developed; two in the left valve, one in the right, with a smaller tooth on either side. Lateral teeth rather short, straight or slightly curved; two in the left valve, one in the right. Beak cavity relatively shallow. Nacre white, iridescent posteriorly.
Habitat: Large rivers in gravel.
Status: Federally Endangered. Most likely extirpated from the Midwest.