Freshwater Mussels Pacific Northwest - The Xerces Society

16
Key Terms
It can be daunting to learn all of the words used to describe the shape and
appearance of mussels. Species identification is a visual process, so we try to
minimize the technical words and illustrate the ones that we use. Please also
refer to the general morphology diagrams on pages 2-3.
Directions and Dimensions
height
length
Left versus right valve: Place the
shell in your palm with the nacre
toward you and the beak up. If
the beak is toward the right, it is
the left valve. If the beak is toward
the left, it is the right valve.
Shell Shape
L:H = ratio of length to height
laterally
compressed
(end view)
laterally
inflated
(end view)
width
Freshwater Mussels of the Pacific Northwest
anterior
(front)
elongate
dorsal
(top)
ventral
(bottom)
beak toward right beak toward left
left valve right valve
posterior
(back)
(L:H = or > 2.0)
subtrapezoidal (L:H = or > 2.0)
elliptical (L:H = 1.5 2.0)
ovate (L:H = 1.5)