Pl. 29, Fig. 8. {104}PINNULARIA VIRIDIS NITZSCH Valve linear-elliptical, with rounded ends; axial area narrow, widened in the middle; striæ, 6 to 7 in 10 µ, crossed by a band as wide as one-third the length of the striæ. Common in fresh water. Pl. 29, Fig. 2. Quite variable in size. Approaches P. major by intermediate forms as in Fig. 9, Pl. 29. PINNULARIA VIRIDIS VAR. FALLAX CL. Valve linear, with rounded ends; axial area narrow, slightly widened in the middle; striæ sometimes unilaterally interrupted, nearly parallel, 10 in 10 µ. Elm, N. J. Pl. 29, Fig. 4. In Fig. 2, Pl. 30, a form is represented which corresponds closely to Navicula viridis var. B, of Wm. Smith. It is given as synonymous with var. fallax; it is bilaterally interrupted. Blue clay. PINNULARIA VIRIDIS VAR. ? Valve linear-elliptical, with rounded ends; axial area narrow, widened in the middle to a transverse fascia which is sometimes unilateral; striæ, 14, in the middle, divergent, convergent at the ends and closer, crossed by a narrow band. L. 45-60 µ. Fascia sometimes absent or very narrow. Northbrook, Pa. Pl. 30, Fig. 17 (represents a form with wider area than usual). PINNULARIA VIRIDIS VAR. CAUDATA N. VAR. Valve elliptical-lanceolate, with sub-rostrate ends; axial area narrow, widened to an orbicular space in the middle; striæ radiate in the middle, 11-12 in 10 µ, convergent and closer at the ends, crossed by a narrow band; median line with very long terminal fissures; terminal nodules noticeable because of the thickening of the edges of the terminal striæ. L. 43 µ. Fresh water, Newtown Square. Not common. Pl. 30, Fig. 18. PINNULARIA SOCIALIS (PALMER) Valve linear, with rounded ends; axial area broad, one-third the width of the valve; striæ slightly radiate in the middle, convergent at the ends, elsewhere parallel, 8 in 10 µ, crossed by an indistinct band about one-third the length of the striæ. L. 60-120 µ. This species, discovered by Mr. Palmer near Media, Pa., is remarkable for the grouping of the frustules "held with girdle sides together by a siliceous cementing of valve edges and enclosed in a common coleoderm." The usual number included in a group is four, but sometimes six or eight are noticed. The frustules adhere near their ends and are so firmly fastened that boiling in nitric acid and bichromate of potash for fifteen minutes will not separate them. _Navicula socialis_ Palmer (Proc. Acad. Nat. Sci., Phila., 1910, p. 460, Pl. 35). Media, Pa. Pl. 29, Fig. 5. {105}PINNULARIA ÆSTUARII CL. Valve linear, with rounded ends; axial area broad, less than one-third the width of the valve; central area a transverse fascia; striæ, 7 in 10 µ, parallel except at the ends where they are slightly convergent; median line flexuose, with short, terminal semicircular fissures. L. 85 µ. Port Penn, Delaware River. Rare. Pl. 29, Fig. 6. GRACILLIMÆ PINNULARIA MOLARIS (GRUN.) CL.