Frustulia weinholdii and Frustulia neomundana are unusual species of Frustulia that possess a set of incomplete longitudinal ribs and unilaterally deflected proximal-raphe ends. This peculiar combination of traits clearly contrasts these species from most congeners, including the generitype. We reexamined material investigated by J. H. Wallace in order to determine diversity within this species group. We compared the specimens from his South Carolina sample with morphologically similar specimens collected in Hawaii, USA. Our investigation of specimens from both Hawaii and South Carolina with light and scanning electron microscopy revealed a previously undocumented amount of morphological diversity in species from the F. weinholdii complex. We describe this morphological variation and present three new species, F. capitata, F. latita, and F. soror, which can be distinguished from similar species by general valve shape, striae density, and longitudinal rib appearance. Presence of incomplete longitudinal ribs and curved raphe ends may be features used to distinguish this group of species from other, typical Frustulia taxa.