The group of bilaterally symmetric diatoms. This is a heterogeneous group that includes araphid, monoraphid, and biraphid taxa.

In older taxonomic systems, pennate diatoms are contrasted with the centric diatoms. However, these two groups are not natural, evolutionary lineages.

Symmetrical Biraphid
Symmetrical biraphid diatoms are considered pennate.
Example of a number of araphid diatoms.