Pl. [18], Figs. 14, 22; Pl. [38], Fig. 16; Pl. [40], Fig. 8.
C. ventricosa is considered by some authors to be equivalent to C. affinis var. semicircularis Lagerst., Encyonema prostratum (Berk.) Ralfs, E. cæspitosum Kuetz. and E. auerswaldii Rab. H. L. Smith's Type Slide of C. ventricosa Ag. is said to equal C. affinis Kuetz., but the specimens appear to me to be equivalent to C. ventricosa Kuetz. Cleve unites many forms, including E. cæspitosum, under C. ventricosa.
CYMBELLA PROSTRATA (BERK.) CL.
Valve semi-elliptical, obtuse at the apices, which are sometimes prolonged and turned downwards; median line straight, terminal nodules distant from the ends; axial area narrow, central area rounded; striæ in radiating, slightly curved lines, indistinctly punctate.
Common in fresh water; occasional in brackish.
Pl. [18], Fig. 21 (represents a frequent variation).
CYMBELLA PHILADELPHICA N. SP.
Valve semi-elliptical-lanceolate, with rounded apices; ventral margin strongly gibbous; terminal nodules distant from the ends; axial area broad, central area widened on the dorsal side; striæ radiate, not curved nor of unequal length, indistinctly punctate, 10 in 10 µ on the dorsal, 8 in 10 µ on the ventral side. L. 86 µ.
This form approaches Encyonema prostratum (Berk.) Ralfs, Schmidt's Atlas, Pl. 71, Fig. 7, but differs in the striæ and the axial and central areas.
Blue clay of Philadelphia. Rare.