On the other hand the nacre color of most species is fairly constant. Of course, if your mussel is living, you will have to kill it to identify the nacre color. Hinge teeth, like nacre color, can only be seen in dead shells, but they are a good diagnostic feature. The length of the teeth, their thickness, how they are oriented, and whether they are arched are key characteristics. Some groups, like Anodonta , lack teeth altogether.
Sculpture is often bewilderingly variable between populations.
In one river individuals may be covered with coarse bumps or knobs, while in another the sculpture will be all but absent. Beak sculpture is the most consistent feature, but also the most elusive. Many species have post-metamorphosed juveniles with a distinct sculpture, called beak sculpture. This sculpture often is not present on any but this earliest part of the shell. Unfortunately, wear and tear, plus acids in the water, usually abrade and dissolve this portion of the shell away.