Unicellular or filamentous. Cells either free, sessile, united in filaments, immersed in a gelatinous envelope or in fronds composed of branching tubes; microscopic, enclosed in a more or less siliceous envelope (frustule), composed of two parts (valves), usually connected by an intervening band (zone or girdle). Cell contents include yellowish or brownish chlorophyll-like bodies which occur in one or several bands (placcochromatic), or as variously distributed granular masses (coccochromatic) lining the inner walls. Growth by ordinary cell division or by auxospores; sexual multiplication by the formation of sporangia. Valves of two kinds: (a) Those in which the markings or parts are more or less concentric (Centricæ); (b) Those (Pennatæ) in which the parts are more or less symmetrically divided by a line (pseudoraphe) or by a cleft (raphe).
CENTRICÆ
Valves without a dividing line or cleft; markings more or less radiate; transverse section of frustule circular, polygonal, or elliptical, sometimes irregular.
Divided into four groups:
1. Discoideæ.—Frustules (cells) discoid; valves without horns or elevations (sometimes with processes).
2. Solenoideæ.—Frustules with numerous girdle bands.
3. Biddulphioideæ.—Frustules box-like, i. e., with the longitudinal axis greater than in the Discoideæ. Valves with two or more angles, elevations or horns.
4. Rutilarioideæ.—Valves as if naviculoid, but with irregular or radial structure.
Groups 2 and 4 are not included in our description. No. 2 contains plankton genera only, while No. 4 consists of genera not yet found in this locality.