Cells of Pyrophacus steinii (Schiller) Wall et Dale are round and lens
shaped and have and anteroposteriorly compressed theca. The epitheca
has a truncated, conical horn and a hexagonally shaped apical pore pla
te with two arched slits positioned off center. The cingulum is equato
rial, narrow, and deep. The hypotheca is flat. The sulcus is narrow, s
lightly curved, and recessed and does not reach the cell's antapex. Th
e plate formula in these specimens of P. steinii is Po, 8', Oa, 13 '',
13C, 12''', 3p, 3'''' and 8S with a difference in the number of preci
ngular (13 '') and postcingular (12''') plates. No additional posterio
r intercalary plates were present (Oap). Pregametic stages of P. stein
ii were observed during cell division via binary fission, with formati
on of two cells and multiple division with formation of four and eight
cells. These newly formed cells were pale in color and were enclosed
in double-layered hyaline membrane. Gametes with gymnodinoid morpholog
y were observed within the parental cells. Planozygotes are large and
round and enclosed in double-layered hyaline membrane. Mature cell for
ms are brown with a microgranular cytoplasm, storage bodies, and a red
accumulation body. The hypnozygote exhibits triple-layered hyaline me
mbrane, irregularly shaped and comparable with bulbous processes of Tu
berculodinium vancampoae Rossigol resting cysts. Division within a hyp
nocyst of P. steinii involves shedding the parental theca and the deve
lopment and emergence of two daughter cells with the size and morpholo
gy of pregametic cells.