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Abstract
Examination of a single fossil specimen from the genus Tetracyclus showed that it is a post auxospore valve of the species Tetracyclus celatom. Using methods similar to those used in living populations, the relationship of the auxospore to its vegetative valves was determined by proposing an ontogenetic relationship between isolated valves in the deposit. The identity of Tetracyclus ellipticus var. constricta, with which the above auxospore of T.celatom could be confused, was ascertained from examination of type material; this diatom belongs to a different species, related to Tetracyclus ellipticus var. lancea f. lata. The importance of establishing species ontogeny for accurate characterisation of taxa is stressed.