Knowledge of the following terms is important for understanding the 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
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