Valves differ from each other (only one has a raphe; i.e. the genus is monoraphid). Valves arched in girdle view. Raphid valve usually has a band of thickened silica running across its centre. The pictured species (A. inflata) often forms short chains.
Achnanthes, Lake Waihola, X640. Photo: Otago Regional Council & Manaaki WhenuaAchnanthes, X640. Photo: Otago Regional Council & Manaaki Whenua