Coscinodiscus guide
  1. Valves circular
  2. Areolae radiate from central annulus
  3. Areolae loculate
  4. Rimoportulae present

The frustules of extant Coscinodiscus species are discoid, barrel-shaped, or slightly eccentric in girdle view. Cells occur singly except immediately following division and can be quite large (up to 500 µm in diameter). The valve face is flat or lens-shaped, with radiate areolae that extend from a central annulus. The areolae are loculate. Externally, areolae are covered by porose vela. Internally, areolae are open to the cell interior via foramina. This arrangement is opposite to the pattern observed with similarly-shaped Thalassiosira species, where the foramina are external and vela are internal.

Marginal rimoportulae are arranged at regular intervals around the periphery of the valve face. The rimoportulae are of two sizes – macro-rimoportulae and micro-rimoportulae. The latter of these being more numerous and regularly arranged. Externally, the rimoportulae have simple, round openings. Internally, the processes may be elaborately developed and extend into the cell interior.

Fultoportulae are absent in this genus. In contrast, Thalassiosira possesses fultoportulae.

Coscinodiscus is predominantly a marine genus with over 250 currently-accepted species names (M.D. Guiry in Guiry, M.D. and Guiry, G.M. 2015).