In conjugation, two frustules form two cylindrical bodies which later become conical and from which are formed the sporangial valves twice the usual size.
FRUSTULIA LEWISIANA (GREV.) DE TONI
Valve elliptical or linear, with rounded ends; terminal nodules elongated, at a distance from the ends; striæ, 24 in 10 µ.
Port Penn, Delaware River. Along the coast.
Pl. [17], Fig. 1.
FRUSTULIA RHOMBOIDES (EHR.) DE TONI
Valve lanceolate or rhombic-lanceolate, rounded at the ends; central and terminal nodules short; striæ, 20 in 10 µ, sometimes coarser.
Common in fresh water.
Pl. [17], Fig. 2.
FRUSTULIA RHOMBOIDES VAR. AMPHIPLEUROIDES GRUN.