Anomoeoneis costata

Valves are rhombic-lanceolate with broadly rounded apices. The axial area is wide and bordered on each side by a curved longitudinal column of areolae. The central area is asymmetric, the wider side reaching the valve margin. Ghost striae are often visible in the central area. Raphe branches are lateral and straight, becoming filiform towards the distal ends. Distal raphe fissures are hooked sharply to one side. Proximal raphe ends have tear-drop shaped pores. Striae are weakly radiate, becoming parallel then weakly convergent at apices. Areolae are irregularly spaced, except at the valve margins.

Four forms of A. costata have been recorded from Montana waters (see Autecology section below). These may be ecomorphs or, more likely, they are neoendemic species that evolved independently in isolated bodies of water. (To an aquatic organism, a lake is an island in a sea of land and a place where island biogeography applies.) Here we describe the most common and most typical form of A. costata, the one with rhombic-lanceolate valves.

This taxon has also been reported as a form and as a variety of Anomoeoneis sphaerophora. Based on culture experiments, Schmid (1979) claimed that both A. sphaerophora f. costata and A. sphaerophora f. sculpta are intermediate stages in the life cycle of A. sphaerophora. However, photographic evidence of these transition stages is lacking and until these life cycle intermediates can be replicated in other laboratories from pure cultures of A. sphaerophora, we elect to report A. costata here as a species.