Maximum spreads of epicontinental seas in the area now covered by the USSR correlate closely with major periods of diversification of benthic calcareous algae (especially chlorophytes and rhodophytes) during the early-middle Ordovician and the late Devonian-early Carboniferous. This is consistent with the 'species-area' concept of direct relationship between taxonomic diversity and habitat-size. The shape of the Earth's hypsometric curve makes it likely that continental flooding would enlarge the area habitable by benthic algae more than it would for benthos not limited to the photic zone, thus enhancing the species-area effect for algae at times of sea-level change. In contrast, the first occurrence of calcareous algae near the Precambrian-Cambrian boundary does not seem to correlate with a transgressive maximum. Cyanophytes are the main calcified group appearing in the early Cambrian and it is possible that their occurrence at this level reflects a different factor which resulted in the calcification of pre-existing algal groups.