Section 1.--Costæ of Nearly Equal Width Throughout, Reaching the

pseudoraphe.

Section 2.--Costæ short or marginal.

Section 3.--Costæ dilated at the margin, attenuated toward the pseudoraphe.

Section 4.--Valve having the appearance of Nitzschia, with inconspicuous alæ (Stenopterobia).

The endochrome consists of two laminate chromatophores, one on each valve.

The auxospores are single, originating from the union of two frustules (H. L. Smith).