Pl. [19], Fig. 23.
GOMPHONEMA BRASILIENSE VAR. DEMERARÆ GRUN.?
Valve lanceolate, with sub-cuneate apex and narrowed basis; axial area lanceolate, broad; no stigma; median fissures remote; striæ parallel, 12 in 10 µ, punctate, the puncta obsolescent, small or interrupted.
Willistown, Pa. Rare.
Pl. [19], Fig. 24.
Pleurosigma WM. SM. (1852)
(pleura, a side, and sigma, the letter s)
Valve lanceolate, sigmoid; axial area very narrow, central area small; striæ punctate, in transverse and oblique lines.
Cleve divides the forms usually known as Pleurosigma into two genera, Pleurosigma and Gyrosigma. Pleurosigma includes all forms having oblique rows of puncta, while Gyrosigma includes all having longitudinal rows. Both have transverse striæ. The former consists entirely of marine species, while in the latter the species are found in fresh, brackish and salt water.
The endochrome in Pleurosigma, according to Mueller, consists of two bands which differ in the median part of each valve. Mereschkowsky says that the endochrome is so divided as to form four bands, two on each valve, that their position is different in different species, and that they are not the same on valves of the same frustule.