REVIEW OF THE NORTH-AMERICAN AQUATIC SNAIL GENUS PROBYTHINELLA (RISSOOIDEA, HYDROBIIDAE)

The aquatic gastropod genus Probythinella THIELE is redefined and prop osed as a monophyletic subunit (sister to Cincinnatia) within the Nort h American component of the family Hydrobiidae (Prosobranchia: Rissooi dea) on the basis of synapomorphies of blunt shell apex, presence of t wo seminal receptacles, short albumen gland, and blunt terminus of pen is. Probythinella is composed of two parapatric species that are disti nguished by derived modifications of the outer edge of the operculum a nd pigmentation of the pallial roof and cephalic tentacles, and also d iffer in shell shape, thickness of outer shell lip, size of buccal mas s (relative to shell height), and number of cusps on the lateral and m arginal radular teeth. Species recognized are Probythinella emarginata (KUSTER), distributed throughout the Mississippi River drainage and d rainages of Great Lakes, Hudson Bay, and Mackenzie River; and P. prote ra PILSBRY, found in brackish waters along the eastern Gulf of Mexico.