Quantitative analysis of shape variation in type and modern populations ofMeridion (Bacillariophyceae)

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