A5. Cells enclosed in glassy, ornamental frustules; many species exhibit gliding movement: Diatoms

B1. Cells shaped like a disk or cylinder; circular in cross-section; raphe absent (mostly nonmotile): Centric Diatoms

B2. Cells mostly elongated and bilaterally symmetric; raphe absent (mostly nonmotile): Araphid Diatoms

B3. Valve view asymmetric and often curved; side (girdle) view bilaterally symmetric; raphe weakly developed (minimal gliding movement): Eunotioid Diatoms

B4. Cells bilaterally symmetric in top (valve) and side (girdle) views; well developed raphe on both sides; very large, diverse group: Symmetric Biraphid Diatoms

B5. Cells bilaterally symmetric in top (valve) view; well developed raphe on one side (other side lacks raphe): Monoraphid Diatoms

B6. Cells bilaterally asymmetric in top (valve) or side (girdle) views, or in both views; well developed raphe on both sides; very large, diverse group: Asymmetric Biraphid Diatoms

B7. Cells asymmetric in valve view, with a curved raphe: Epithemioid Diatoms

B8. Cells bilaterally symmetric or asymmetric, but with distinctive raphe that is either twisted or off-set to one side of valve: Nitzschioid Diatoms

B9. Cells bilaterally symmetrical in valve view, but often curved; raphe often located along margin: Surirelloid Diatoms