Blue clay. Not common.
Pl. [10], Fig. 12.
PLAGIOGRAMMA TESSELLATUM GREV.
Valve elliptical-lanceolate; central space transversely elliptical to the major axis, half the diameter of the valve; terminal spaces more or less circular or ovate. Granular markings large, quadrangular, in transverse rows. Pseudoraphe not distinct. As the central space does not reach the margin, it is a question whether this form is a Plagiogramma or a new genus.
Blue clay. Rare.
Pl. [10], Fig. 11.
Opephora Petit (1888)
(ope, an opening, and phoreo)
Frustule rectangular. Valve cuneiform, linear or elliptical-lanceolate, with broad, transverse striæ and a well-defined pseudoraphe or median area.
The genus "portant des stries en forme de boutonnières," as Petit remarks, is quite near Fragilaria, under which the species here described were originally included. (See Schmidt's Atlas, Pl. 298, where numerous forms of F. pinnata are figured.)