The order Pennales is a traditional subdivision of the heterokont algae known as diatoms.
The valves may be linear or oval in shape, and usually bear bilaterally symmetrical ornamental patterns.
These patterns are composed of a series of transverse lines (known as striae) that can appear as rows of dots when viewed with an optical microscope.
Some pennate diatoms also exhibit a fissure along their longitudinal axis.
In some taxonomic schemes,[2] the pennate diatoms are divided into two groups: pennate diatoms without a raphe (a seam or ridge), known as araphids (order Fragilariophyceae), and pennate diatoms with a raphe, known as raphids (order Bacillariophyceae).