The testate amoebid genus Corythionella inhabits beach sands and includes three marine and two freshwater species of the "acolla-complex", all characterized by a poorly developed flared collar surrounding the pseudostomal aperture. Multivariate statistical methods (cluster analysis, principal coordinates analysis, non-metric multidimensional scaling ordination and discriminant analyses) were applied to morphologic characters of the test (shell) of the members of this complex in order to visualize and assess the degree of similarity among them. Graphical displays of the results of these analyses clearly revealed distinct separations between all pairs of species, including a sixth previously un-named taxon described here as a new species (Corythionella darwini n. sp.). Discriminant functions were developed to assist in identification of Corythionella with reduced oral collars. C. darwini was discovered in Pacific Ocean beach sand samples from western Canada and is characterized by a test with a median length, width, height and pseudostome width of 53, 20, 19 and 12microm, respectively. It presents the least variability in test morphology of all known species in the "acolla-complex" (coefficients of variation ranging between 3 and 5.6% for the four test variables) and is further distinguished from the other members of the complex by its distinctive near-cylindrical shape.