Quadrula cylindrica (Say, 1817)
Quadrula cylindrica, INHS 6974. North Fork Vermilion River, Vermilion County, Illinois.
Other common names Smooth cob shell, spectaclecase mussel.
Key characters Elongate, rectangular shell with pustules and V-shaped markings.
Description Shell rectangular, elongate (about three times as long as high), thick, and compressed to moderately inflated. Anterior end rounded, posterior end squared or truncated. Dorsal and ventral margins parallel. Umbos low, only slightly elevated above the hinge line. Beak sculpture consists of two rows of knobs or ridges that continue down the lateral surface of the shell. Surface of the shell usually rough, with numerous tubercles on the anterior end and a series of large pustules or knobs along the posterior ridge. Periostracum green or light brown (darker in older shells) with yellow zigzag or chevron-shaped markings on the shell. Length to 5 inches (12.7 cm).
Pseudocardinal teeth serrated and well developed; two in the left valve, one in the right. Lateral teeth very long and straight; two in the left valve, one in the right. Beak cavity deep. Nacre pearly white, iridescent posteriorly.
Habitat Medium to large rivers in mixed sand and gravel.
Status Rare throughout its range. Endangered in Illinois, Indiana, Missouri, and Ohio.