Guide to Anorthoneis

Anorthoneis is primarily a marine genus of littoral, epiphytic or epipsammic habitats in coastal areas of temperate and warm water. One species, A. dulcis Hein, was described from freshwater rivers in northern Florida. The species has also been reported from rivers in Baja California, Mexico.

The morphological features are documented here for this freshwater species, rather than for the genus as a whole. Valves of A. dulcis are broadly elliptical. The valve mantle is relatively low. The axial area on both raphe and rapheless valves is eccentric, although sometimes only slightly displaced from the center. The sternum does not extend to the poles of the valve. The areolae are arranged in radiate, curved rows on both the raphe and rapheless valve. The striae are uniseriate, becoming biseriate at the valve margin. Living cells of A. dulcis attach to substrata by short, mucilaginous stalks or the cells are adnate in a manner similar to Cocconeis. The stalks were found to originate from different positions of the frustule, rather than at one apex, as in most stalk attachments. Also similar to Cocconeis, the cells possess a single, flat, C-shaped plastid.