Diagnostic features
Valves differ from each other (only one has a raphe; i.e. the genus is monoraphid). Raphid valve is strongly arched in girdle view. Very small, narrow cells. Araphid valve still has a space between the lines of striae (unlike Achnanthes), but this does not contain a raphe. Cells may be attached to surfaces by a mucilaginous stalk.
![[Achnanthidium], north Taranaki swamp, X400. Photo: Taranaki Regional Council & Manaaki Whenua Image](/assets/Tools-And-Resources/Identification/algae/achnanthidium400__ScaleWidthWzEwMjRd.jpg)