A structure that is divided into two branches is said to bifurcate.
Pennate diatoms with a raphe system on both valves of the frustule are biraphid. For example, Amphora, Epithemia, Gomphonema, Navicula and Mastogloia are all biraphid genera. In contrast, some pennate diatoms lack a raphe completely (i.e., Diatoma), while others have a raphe system on only one valve (monoraphid), such as Planothidium.
Striae that are composed of two rows of areolae are termed biseriate.