Valve with margins parallel nearly to the extremities, which are suddenly unilaterally sub-conical and obtuse; raphe sigmoid; transverse and longitudinal striæ nearly equally distant, 15 in 10 µ (Per.). L. 200-360 µ. _Navicula baltica_ Ehr. _Pleurosigma balticum_ (Ehr.) Wm. Sm. Common along the coast. Pl. 23, Fig. 2. GYROSIGMA PARKERI VAR. STAURONEIOIDES GRUN. Valve lanceolate, slightly sigmoid, ends produced into beaks with sub-acute apices; raphe straight in the middle part; central nodule elliptical; transverse striæ, 21, and longitudinal, 24 in 10 µ (Per.). An apparent stauros, variable in width, extends to the margin and, in consequence, the median transverse striæ are more evident. L. 75 µ. Schuylkill River. Rather rare. Pl. 23, Fig. 7. {76}GYROSIGMA SIMILE (GRUN.) Valve slightly sigmoid, broad, with obtuse ends; raphe sigmoid, nearly central; transverse striæ, 15, longitudinal, 16-17 in 10 µ (Per.). _Pleurosigma simile_ Grun. _Gyrosigma balticum_ var. _similis_ (Grun.) Cl. Shark River, N. J. Pl. 23, Fig. 4. (3) TRANSVERSE STRIÆ MORE DISTANT GYROSIGMA ACUMINATUM (KUETZ.) CL. Valve sigmoid, tapering to the sub-acute ends; raphe central; transverse and longitudinal striæ nearly equally distant, 17 or 18 in 10 µ (Per.). _Frustulia acuminata_ Kuetz. Port Penn, Delaware River. Pl. 23, Fig. 5. GYROSIGMA STRIGILIS (WM. SM.) CL. Valve sigmoid, with obtuse ends; raphe doubly sigmoid; axial area rather wide; transverse striæ, 13, and longitudinal, about 16 in 10 µ. Long Island Sound. Not common. Pl. 23, Fig. 1. GYROSIGMA KUETZINGII (GRUN.) CL. Valve sigmoid, lanceolate, with sub-acute ends; raphe central, the central nodule elliptical; transverse striæ, 21-23, and longitudinal, 25-26 in 10 µ. _Pleurosigma spencerii_ var. _acutiuscula_ Grun. _Pleurosigma spencerii_ var. _kuetzingii_ Grun. Common in fresh water. Pl. 38, Fig. 12. GYROSIGMA SCALPROIDES (RAB.) CL. Valve slightly sigmoid, with obtuse ends; raphe nearly straight; central nodule elliptical; transverse striæ, 22, slightly radiate and more distant in the middle; longitudinal striæ, 29 in 10 µ. L. 60 µ. Common in streams. Pl. 38, Fig. 9. In Pl. 23, Fig. 6 represents a form more sigmoid. GYROSIGMA SPENCERII VAR. NODIFERA GRUN. Valve sigmoid, with obtuse ends; raphe central; central nodule obliquely elongated; transverse striæ, 17-18 in 10 µ, curved in the middle of the valve, longitudinal striæ, 22 in 10 µ. L. 150 µ. Blue clay. Pl. 23, Fig. 8. GYROSIGMA PROLONGATUM (WM. SM.) CL. Valve narrow, lanceolate, produced into beaks, curved in a contrary direction; raphe central; transverse striæ, 20-21 in 10 µ, longitudinal closer. L. 140 µ. Along the coast, northward. Pl. 38, Fig. 13. I have not seen any specimens south of New England, but they will probably occur. {77}(4) STRIÆ ALIKE, EXTREMITIES PRODUCED GYROSIGMA FASCIOLA (EHR.) CL.