Cyclostephanosspecies newly reported from North America:C. tholiformissp. nov. andC. costatilimbuscomb. nov.

Abstract
A new species, Cyclostephanos tholiformis Stoermer, Håkansson & Theriot, and a new combination Cyclostephanos costatilimbus (Kobayasi & Kobayashi) Stoermer, Håkansson & Theriot are proposed. These species are abundant in a eutrophic, hardwater, habitat in central North America. They resemble other small species of Stephanodiscus and Cyclostephanos commonly found in such habitats, and can be distinguished from them with certainty only on the basis of characters visible with electron microscopy.