Valve lanceolate, attenuated into curved beaks turned in opposite directions; raphe central, straight, except at the beaks; transverse striæ, 22, longitudinal, 24 in 10 µ (Per.). New York Bay. Pl. 23, Fig. 9. FRUSTULIA AG. (1824); em. GRUN. (1865) (frustulum, a small piece) Valves naviculoid, similar, usually free but sometimes enclosed in gelatinous tubes or embedded in mucus. Median line between two thickened ribs. Central and terminal nodules frequently elongated. Surface of valve with fine puncta in longitudinal and transverse lines appearing hyaline under medium powers. Chromatophores, two, extending along the girdle. They differ from those of Navicula in being separated from the wall in the middle by a hemispherical mass of protoplasm. According to Pfitzer, each chromatophore is divided in the middle, allowing a connection between the hemispherical mass and the central plasma mass. Schmitz states that the chromatophore is thickened in the middle and contains a pyrenoid. 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. Valve rhombic-lanceolate; central and terminal nodules elongated; median line somewhat excentric. Blue clay. Pl. 17, Fig. 3. FRUSTULIA RHOMBOIDES VAR. SAXONICA (RAB.) DE TONI Valve smaller than in rhomboides, with somewhat produced ends, closer median ribs and rounded central nodule. Fresh water. Pl. 17, Fig. 6. FRUSTULIA VULGARIS (THWAITES) DE TONI Valve elliptical-lanceolate, with rounded or sometimes sub-rostrate ends; central and terminal nodules slightly elongated; striæ delicate, closer at the ends. Frustules at first in gelatinous tubes. _Colletonema vulgaris_ Thwaites. Fresh water. Pl. 17, Fig. 4. {78}FRUSTULIA INTERPOSITA (LEWIS) DE TONI Valve linear-elliptical, rounded at the ends; terminal nodules short. _Navicula interposita_ Lewis. Along the coast. Port Penn, Delaware River. Pl. 17, Fig. 5. AMPHIPLEURA KUETZ. (1844) (amphi, on both sides, pleura, a side) Frustules free, in gelatinous masses or in tubes. Valve linear-lanceolate; central nodule narrow, extending half the length of the valve or more, then forking toward the ends. Terminal nodules prolonged, as in Frustulia, into a "porte-crayon-shaped" figure. Chromatophores two, very short. AMPHIPLEURA PELLUCIDA KUETZ.