Chydorus bicornutus and C. faviformis are presently confined to the glaciated region of the northeastern United States and adjacent Canada where they quite commonly occur together. Chydorus bicollaris n. sp., in contrast, is presently confined to the Coastal Plain from Florida to New Jersey, where it just barely overlaps C. bicornutus and can co-occur with it. Chydorus faviformis and C. bicollaris are completely allopatric. A redistribution of all three species during successive glaciations and interglaciations is postulated.Chydorus bicornutus differs markedly from other species included in Chydorus. Chydorus bicollaris is somewhat more like Chydorus, but differs significantly in characters shared by members of the sphaericus complex. Chydorus faviformis quite closely resembles members of the sphaericus complex, including the male, which loses its honeycomb pattern on reaching maturity. These marked differences in morphology suggest that the three species are less closely related to one another than their sharing of a honeycombed exterior might suggest.Scanning electron micrographs of specimens in process of molting show the honeycomb pattern of the new exoskeleton collapsed and containing many complex wrinkles, which disappear as the animal subsequently expands. The functions of this bizarre exterior are unknown, including the vast difference in size of the lateral horns of bicornutus.