Achnanthidium Rosenstockii Guide
  1. Apices subcapitate
  2. Central inflation
  3. Raphe valve with 2 widely spaced striae at center
  4. Rapheless valve with lancolate axial area

Valves are linear-lanceolate, with a central inflation equal or larger in width to the subcapitate ends. The axial area is large and lanceolate on the rapheless valve. The raphe valve is characterized by two widely spaced striae at center valve.