Placoneis abiskoensis has linear valves, with subcapitate apices. Placoneis elginensis and P. paraelginensis have capitate apices (in larger specimens) and the distal raphe ends are unilaterally deflected. Placoneis placentula has wide (12-20 µm) elliptic valves, with acutely rounded apices. Placoneis symmetrica has rostrate to capitate apices, radiate striae throughout and a higher areola density (28 in 10 µm).