Valve obovate-lanceolate or nearly linear with rounded apices; striæ transverse, broad, 3 or 4 in 10 µ; median area lanceolate.
An inner stratum, with puncta in transverse rows, is apparent.
Blue clay. Not uncommon. Variable in size.
Valve linear, oblong, with rounded apices. Median area linear, narrow; striæ punctate.
Petit (Diat. Cap Horn) in his diagnosis states that the valves are cuneiform, but they are not always so.
{ 44}OPEPHORA PINNATA VAR. LANCEOLATA N. VAR.
Valve lanceolate; costæ slightly radiate, punctate; median area broad, lanceolate.
Differs from O. pinnata in outline, radiation of the costæ and median area.
_Fragilaria._--Frustules in fasciæ. Valve with transverse striæ. Pseudoraphe indistinct.
_Rhaphoneis._--Striæ radiate; pseudoraphe distinct.
_Asterionella._--Frustules in star-shaped clusters.
(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.
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.
Valve oblong or elliptical, 10 µ in length; striæ subtle, with coarse, short striæ at intervals on the margin and evident in zone view.
Valve in general outline linear-elliptical, with extremities produced; striæ subtle; pseudoraphe distinct.
Valve linear, with rounded apices; striæ subtle; pseudoraphe indistinct.
Pl. 10, Fig. 37. Fig. 36 is an indeterminate form occasionally found in the blue clay.
Valve linear, constricted at the hyaline middle; apices slightly produced; striæ, 17 in 10 µ. Quite variable in size.