A list of those species similar in color, shape, and size is included for comparison, when some doubt as to the identity remains. The species listed may or may not be closely related. The descriptions are for shells only and do not include any of the characteristic features of the animal itself. The first paragraph describes the external parts of the shell, including coloration, shape, size, surface features, and thickness. The second paragraph covers internal features, including the pseudocardinal teeth, lateral teeth, depth of the beak cavity, and nacre color.
Mussels vary considerably with respect to their habitat preferences.
Over half of the species in this field guide are considered rare, threatened, or endangered by at least one of the states in the Midwest. Each account gives a general statement regarding the species distribution and abundance, often followed by information on its status in individual states. Not all states use the same criteria in determining or labeling special-status species. This field guide also employs several categories used by the U.S. Department of the Interior. The shells were selected because they were considered representative of their species.