Diatoma Guide
Credit: Marina Potapova
  1. Sternum may be discernable, or not
  2. Transapical ribs heavily silicified
  3. Spines absent, or scattered at porefields
  4. Rimoportula present

The genus Diatoma possesses a raised central sternum, which may or may not be distinct. Transverse costae are characteristically thickened and septae are absent. Striae are comprised of uniseriate rows of areolae. Each valve has a single rimoportula, positioned near a valve terminus and oriented transapically. A ligula is present on the second girdle band, which attaches to the polar position of the valve, adjacent to the interlocking valvocopula. Spines are scattered near the polar porefields, but are absent elsewhere. Living cells contain numerous discoid, or platelike plastids.

J├╝ttner et al. (2015) propose that Diatoma and Odontidium, previously thought of as sub-generic sister taxa, should be considered separate genera. Together, Diatoma and Odontidium comprise one of the two monophyletic araphid groups to evolve heavily silicified transapical ribs. The other group includes Meridion, Distrionella, Tetracyclus and Tabellaria (with the ribs being lost in Tabellaria).