STUDIES ON THE GENERA GYROSIGMA AND PLEUROSIGMA (BACILLARIOPHYCEAE). PLEUROSIGMA OBSCURUM W. SMITH REVISITED

Abstract

The claim that Pleurosigma obscurum W. Smith should be assigned to the genus Gyrosigma (Reid 1998a) was based on irreproducible findings. It would require that the prime generic characters of Pleurosigma and Gyrosigma (areolar arrangement in 3 obliquely and 2 perpendicularly intersecting linear systems respectively) be abandoned. Although the two oblique systems in Pleurosigma obscurum are so fine as to be beyond resolution in the LM, EM examination of the type material and specimens from other populations unequivocally reveals a 3–system arrangement precluding assignment to Gyrosigma.