Abstract
Nine new species (Unionicola (Atacella) neoperforata, petita, recta, crassiparma, parmaphora, nelsoni, quadriplaca, rosewateri and redfordi) are described for the first time. The first two species are from Mexico, and the latter seven are from a single collection in Brazil. All species are mussel parasites and exhibit unique features which expand the current concept of Atacella and exemplify evolutionary adaptive radiation. U. (A.) entrerrianensis (Rosso de Ferradas) and U. (A.) fissipes (Koenike) are reported from Mexico and Honduras, respectively.