The family Margaritiferidae is commonly referred to as pearl mussels, naiads, or margaritiferids. No more than 15 species in 3 genera have been described, which is nearly 5% of the Unionoida species in the Holarctic (Smith, 2001b).
In general, Margaritiferidae are acephalic (no head), bivalved mollusks usually with the beak (the elevated portion of the dorsal margin) slightly anterior. When present, the pseudocardinal teeth are anterior to the beak and the lateral teeth are posterior. The species in this family have a foot rather than a byssus, fibrous structures found in other mussel families. Along with Unionidae, another family included in the order Unionoida, Margaritiferidae does not have true siphons. Unlike the family Unionidae, the inhalant aperture (opening in the posterior end of the mantle border where water enters the mussel) of Margaritiferidae has branched papillae (bumps). The shells are elongate, thick, black, rhomboidal and often arcuate (arched) and range in size from 80 to over 200 mm in length (Smith, 2001b).