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.
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.