Subtle differences in valve morphology can indicate sexually isolated populations in diatoms (Bacillariophyceae). Shape descriptors, like Legendre coefficients, have been used to quantify differences in valve outline so that morphologically distinct groups can be recognized. Legendre coefficients were used as shape descriptors to quantify differences in valve outline among five populations in the genus Meridion Ag. Two novel populations of Meridion from North America, a North American population of M. constrictum Ralfs, type material for M. circulare (Greve.) Ag., and type material of M. constrictum were included in this analysis. Results of a principle components analysis and discriminate analyses on the shape descriptors of all five populations support the hypotheses that the two novel North American populations are undescribed taxa within Meridion and that the North American M. constrictum can be considered Meridion constrictum sensu stricto. A new taxon, Meridion hohii Rhode, is described. There appears to be greater diversity within the genus Meridion than is currently recognized.