Platessa Guide
Credit: Marina Potapova
  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Valves nearly flat
  3. Valves elliptic, linear-elliptic to elliptic-lanceolate
  4. Striae mostly biseriate, sometimes uniseriate on raphe valve
  5. Terminal raphe fissures absent

Frustules are heterovalvar, one valve possesses a raphe and the other valve lacks a raphe. Valves are elliptic, linear-elliptic or elliptic-lanceolate. Both of the valve faces are nearly flat, in contrast to other monoraphids that have convex and concave valve faces. Striae are mostly biseriate, but in some species the raphe valve face may be uniseriate. Terminal raphe fissures are absent. Instead, the raphe ends with distinct terminal pores. Platessa lacks terminal deflections or hooks. Girdle bands are narrow and lack ornamentation. Cells form single cells or short chains.

The genus Platessa was established (Lange-Bertalot 2004) and contains the former Achnanthes conspicua Mayer and species within the A. holsatica Hustedt group. Members of this genus are widely reported, but low in abundance in collections. Until recently, species within Platessa had misidentified or largely overlooked in North American surveys.