PENNATÆ

Valve zygomorphous. Structure pinnate, not concentric. Valve divided either by a true raphe or cleft or by a linear space or line imitating a raphe.

Divided into three Groups:

1. Fragilarioideæ.—Valves without a raphe; usually with a pseudoraphe or median line.

2. Naviculoideæ.—Either one or both valves with a true raphe.

3. Surirelloideæ.—Valves in which the raphe is concealed near the margin on one or both sides of each valve in a more or less elevated keel or wing.

FRAGILARIOIDEÆ

(a) Tabellarieæ.—Valve symmetrical with respect to both the longitudinal and transverse axes; septate, not cuneate.

(b) Meridioneæ.—Valve symmetrical with respect to the longitudinal axis, asymmetrical to the transverse axis, cuneate, finely striated.