Synedra.—Valve elongate.
Asterionella.—Frustules in star-shaped clusters.
Fragilaria (Lyng.) Rab.
(fragilis, because of the fasciæ easily breaking up)
Frustules rectangular, adnate in fasciæ, soon breaking up. Valve lanceolate, oblong or elliptical in general outline, with convex or sinuate margins; without costæ; pseudoraphe narrow or indistinct; striæ transverse. Chromatophores vary according to species. In some they consist of four bands on the valves; in others they are granular (Mereschkowsky).
Brun divides the genus into two sections, Fragilaria proper and Staurosira. The former, with an indistinct pseudoraphe, includes the species virescens, arctica, undata and linearis, while the latter, with distinct pseudoraphe, includes capucina, harrisonii, construens and parasitica.
FRAGILARIA VIRESCENS RALFS
Frustules in long fasciæ. Valve elliptical-lanceolate, obtuse at the apices; pseudoraphe indistinct; striæ, 17 in 10 µ, punctate.
Very common in springs and pure streams. The fasciæ are often a foot or more in length.
Pl. [10], Figs. 20 and 21.