This description is from Krammer's Caloneis tenuis, which he has as a synonym to P. tenuis. Valve linear to valve-elliptica. Sides parallel to triundulate sides. Ends are slightly distinguishable, bluntly rounded, and slightly capitate. Raphe weakly bent; raphe axis in middle somewhat lateral to philiform, bent at ends. Central nodule is indistinct. End pole nodules are distinctly visible, they are large and bayonet shaped. Axial area is variable, from very thin, linear to very broad, lanceolate-like. Central area variable, from small, circular to large rectangular. Commonly asymmetrical fascia. Striae in the middle are parallel to weakly radial, at the poles are parallel to lightly convergent
(Krammer, K. and Lange-Bertalot, H. 1986).
Cosmopolitan, especially in the northern alpine areas. Found in standing or flowing water. Oligotrophic water with low to moderate electrolyte content(Krammer, K. and Lange-Bertalot, H. 1986; Beaver 1981).