The Diatomaceæ of Philadelphia and Vicinity

by Charles S. Boyer

Blue clay.

Pl. [18], Fig. 5.

CYMBELLA TUMIDA (BRÉB.) V. H.

Valve cymbiform, with gibbous ventral margin and abruptly rostrate ends; median line arcuate; axial area narrow; central area large, orbicular; below the central nodule is a punctum; striæ punctate.

Crum Creek.

Pl. [18], Fig. 7.

Section 3.—Encyonema. Frustules in tubes

CYMBELLA VENTRICOSA KUETZ.

Valve lunate, with straight or slightly gibbous ventral margin; axial area indistinct; median line straight or nearly so; striæ punctate.

Very common, but extremely variable. The ventral margin is sometimes straight and sometimes quite gibbous.