Order Eulamellibranchia Edges of the mantle generally united by one or two sutures.
Mutelidae.-Differs from Unionidae in having two pallial sutures; freshwater.
Mantle not extensively closed; two pallial sutures and two well developed siphons.
Two pallial sutures, siphons somewhat elongated and partially or wholly united.
Asymmetrical, inequivalve, fixed, with extensive pallial sutures; no siphons.