Odontidium Guide
Credit: Ian Bishop
  1. Sternum distinct
  2. Transapical ribs heavily silicified
  3. Spines simple
  4. Girdle structure complex
  5. Rimoportula present

Frustules of Odontidium have heavily silicified transapical ribs that extend internally from the valve face. The sternum is distinct, and may be relatively broad, or may be narrow. Spines are simple and located at the valve face/mantle boundary. Spines are considered non-functional. Girdle band structure is complex, with many alternating open copulae and a pleural band. The copulae possess porose ligulae. One rimoportula located sub-apically on each valve, positioned in-line with a single stria and replacing several vimines. Living cells contain multiple elliptic lobed plastids.

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