Pl. [31], Fig. 24.
SURIRELLOIDEÆ
The Surirelloideæ are usually understood to include the genera Surirella, Podocystis, Cymatopleura and Campylodiscus, all of which resemble each other more or less, either in having a keel or markings like the divisions of the keel in Surirella and a median line, or pseudoraphe. The genus Nitzschia also has a keel, but it does not border each side of the valve as in Surirella, being found either near one margin or between it and the centre. Certain of the Surirellæ are allied to the group Tryblionella of the Nitzschiæ, while forms of Stenopterobia are distinguished with difficulty from the group Sigmata.
The following arrangement, therefore, is intended to include all genera having a keel or something which resembles it.
Hantzschia.—Valve asymmetrical; keels of the two valves opposite each other.
Nitzschia.—Valve asymmetrical; keels not (usually) opposite each other.
Surirella.—Valve usually symmetrical; a keel on each border.
Cymatopleura.—Valve without an elevated keel, but with markings like those of Surirella; undulated in zone view.
Campylodiscus.—Valves saddle-shaped.