PLEUROSIGMA NAVICULACEUM BRÉB. Valve lanceolate, slightly sigmoid at the extremities; raphe strongly sigmoid near the margin at the ends; central nodule large, rounded; oblique striæ, 13-14 in the middle, closer at the ends; transverse striæ, 18-20 in 10 µ (Peragallo). Long Island Sound. Pl. 22, Fig. 6. PLEUROSIGMA VIRGINIACUM H. L. SMITH Valve slightly sigmoid, with acute ends; raphe more sigmoid than the valve, excentric near the ends; oblique striæ in different directions at the centre, 13 in 10 µ, closer and less distinct at the ends; central nodule small but prominent because of its thickness, producing by diffraction an apparently wide area (somewhat exaggerated in the figure). L. 95 µ, usually larger. _P. affine_ var. _fossilis_ Grun. (Peragallo). _P. normanii_ var. _fossilis_ Grun. (Cleve). Common in the blue clay. Pl. 22, Fig. 8. (3) OBLIQUE STRIÆ 60 DEGREES PLEUROSIGMA ANGULATUM (QUEKETT) CL. Valve rhomboidal, with sub-rostrate or produced ends; central nodule rhomboidal; raphe central; transverse and oblique striæ at an angle of 60 degrees, equidistant, 18-22 in 10 µ. _Navicula angulata_ Quekett. Along the coast. Pl. 22, Fig. 3. PLEUROSIGMA STRIGOSUM WM. SM. Valve lanceolate, with sub-acute, somewhat revolute, apices; oblique striæ at an angle of about 60 degrees, otherwise as in angulatum. Along the coast. Not common. Pl. 22, Fig. 1. PLEUROSIGMA ÆSTUARII BRÉB. Valve lanceolate, with produced apices; raphe less sigmoid than the valve and excentric; oblique striæ, 19-21 in 10 µ, at an angle of about 60 degrees. Along the coast. Common. Pl. 22, Fig. 7. {75}(4) OBLIQUE STRIÆ 60 DEGREES, THE TRANSVERSE MORE DISTANT PLEUROSIGMA RIGIDUM WM. SM. Valve nearly straight or slightly sigmoid, with obtuse ends; raphe central, excentric near the ends; oblique striæ, 17-21, transverse, 16-19 in 10 µ. (Peragallo). New Rochelle, N. Y. Pl. 22, Fig. 2 (very near the var. gigantea Grun.) GYROSIGMA HASSALL (1845) (gyros, curved, and sigma) Valve lanceolate, sigmoid; axial area very narrow, central area small; striæ punctate, in transverse and longitudinal rows. Chromatophores two, in long and narrow bands, perforated, differing from those of Pleurosigma. The elæoplasts are also arranged differently in the two genera. (Mereschkowsky, Études sur l'Endochrome des Diatomées, Imperial Academy of Petrograd, 1901, Vol. 11, No. 6, p. 18 et seq.) The arrangement is according to Peragallo. (1) LONGITUDINAL STRIÆ MORE DISTANT THAN THE TRANSVERSE GYROSIGMA HIPPOCAMPUS (EHR.) Valve lanceolate, sigmoid, with obtuse ends; raphe nearly central; transverse striæ 15-17, longitudinal, 10-12 in 10 µ. _Navicula hippocampus_ Ehr. _Pleurosigma hippocampus_ (Ehr.) Wm. Sm. _Gyrosigma attenuatum_ (Kuetz.) Cl. Long Island Sound. Pl. 23, Fig. 3. (2) LONGITUDINAL AND TRANSVERSE STRIÆ NEARLY EQUAL GYROSIGMA BALTICUM (EHR.) CL.