Organic thalli of Ordovician calcareous algae Vermiporella fragilis Stolley and Palaeoporella variabilis Stolley found for the first time are studied. The structure of V. fragilis is shown to be fundamentally different from that previously described by various authors. Its thallus consists of a central stem and lateral branches of three orders, arranged in regular whorls. Spores are preserved in stem segments, which are separated by septa. Morphologically they resemble the oospores of a Recent alga Sphaeroplea Fritsch. The structure of previously unknown filamentous central parts of thallus of P. variabilis is studied and their relation to subcortical and cortical threads penetrating the calcareous sheath are described. Revised diagnoses of Vermiporella Stolley and Palaeoporella Stolley and of their type species are given.