Rossithidium Petersenii Guide
Credit: Marina Potapova
  1. Valve linear to linear-elliptical
  2. Raphe straight
  3. Striae unseriate
  4. Raphe valve with a large rectangular or transverse elliptical central area
  5. Rapheless valve with a narrowly rhomboid axial area

Valves are linear or linear-elliptical with rounded ends, 3.5-4.5 µm wide, 13-18.5 µm long. Raphe valve has a large transverse elliptical central area that almost reaches the valve margins. Terminal raphe fissures are absent. Rapheless valve has a narrowly rhomboid axial area and no distinct central area. Striae are uniseriate, slightly radiate, and number 32-36 in 10 µm.