Published online by Cambridge University Press: 11 May 2009
A net sample of phytoplankton taken in the outer Moray Firth in which there were many cells of R. hebetata forma semispina and several cells of R. hebetata forma hiemalis (Fig. 1 a, b) was also found to contain cells either with one forma semispina calyptra or one forma hiemalis calyptra, in which the cell wall changed abruptly at the opposite end to a thin weakly silicified, sometimes almost membranous structure, ending in a central, broadly conical apex with a delicate hollow spine (Fig. 2j, k). The perizonium like appearance of one end of these cells suggested that they might be a stage in the development of the auxospore in R. hebetata Bailey (sensu Gran).