Motile thecal cells derived from the hatching of single cysts identified as Spiniferites membranaceus and S. ramosus have been used to establish cultures. These cultures were examined in order to assess the cyst-theca relationships of these two taxa. The cultures produced two different motile Gonyaulax species belonging to Kofoid's Spinifera group. These cultures were then induced to form a new cyst generation under uniform conditions, and examination of large numbers of the resulting cysts has shown that process development is an extremely variable phenomenon although process morphologies display a continuum within a species. Process length (and to a certain degree, process morphology) requires careful interpretation when used to discriminate Spiniferites taxa, in both modern and ancient environments.