Knowledge of the following terms is important for understanding the freshwater musssels species accounts.
Anterior end: the shorter end of the shell as measured from the umbo; also considered the front end
Beak: the raised part of the dorsal margin of the shell; also called the umbo
Beak cavity: the depression or pocket on the inside of each valve leading into the beak
Beak sculpture: the raised loops, ridges, or bumps on the umbo
Chevron: a V-shaped marking
Compressed: flattened or pressed together laterally
Concentric: having a common center
Distal: away from the center or origin
Dorsal: referring to the top part of the shell where the hinge is located
Elliptical: having the form of an ellipse, or oval
Elongate: long or extended
Extirpated: locally extinct
Growth lines: darkened lines on the surface of the shell indicating periods of rest during growth
Hinge: the elastic part of the shell that unites the valves along the top of the shell
Inflated: swollen or expanded
Interdentum: a flattened area between the pseudocardinal and lateral teeth
Iridescent: exhibiting rainbow colors
Lateral teeth: the elongated teeth along the hinge line of the shell
Left valve: the left half of the shell when the dorsal edge or hinge is facing up and the anterior end is directed forward (away from the collector)
Nacre: the interior layer of the shell, usually white, pink, salmon, or purple and iridescent
Oblong: having the shape of or resembling a rectangle or ellipse
Ovate: egg-shaped
Pallial line: a linear depression on the inside of the shell
Periostracum: the outside layer or covering of the shell
Plications: folds
Posterior end: the longer end of the shell as measured from the umbo; also considered the back end; also the end with the siphons, usually sticking above the stream bottom
Posterior ridge: the ridge on the back half of the valve running from the umbo to the posterior ventral edge
Posterior slope: the area along the dorsal part of the shell between the posterior ridges of the valves
Pseudocardinal teeth: the triangular, often serrated, teeth located on the anterior-dorsal part of the shell
Pustule: a bump or raised knob on the outside surface of the shell
Quadrate: square
Right valve: the right half of the shell when the dorsal edge or hinge is facing up and the anterior end is directed forward (away from the collector)
Serrated: notched or grooved
Shell margin: the circumference of a valve outline when laid flat; also known as the edge
Solid: thick or heavy
Striated: marked with lines or grooves
Sulcus: a shallow depression or furrow on the outside surface of the shell
Truncate: having the end shortened or squared off
Tubercle: a pointed, rounded, or knoblike projection on the shell
Umbo: the inflated dorsal part of the shell; also called the beak
Valve: one of the two halves of the shell
Ventral: referring to the bottom edge of the shell