THE DIATOMACE^; OF PHILADELPHIA AND VICINITY 43 



PLAGIOGRAMMA PYGM^SUM GREV. 



Valve linear-elliptical; pseudoraphe distinct; rows of granules transverse, usually six 

 in each compartment, moniliform, three on each side. 

 Blue clay. Not common. 

 PL 10, Fig. 13. 



PLAGIOGRAMMA WALLICHIANUM GREV. 



Valve linear, rounded at the ends; pseudoraphe absent; transverse rows of granules, 

 six or seven in each compartment, and two or three rows of smaller granules at each end. 

 Blue clay. Not common. 

 PL 10, Fig. 14. 



PLAGIOGRAMMA OBESUM GREV. 



Valve rhombic-lanceolate, the costae scarcely visible; pseudoraphe rather wide; rows 

 of granules, about seven in each compartment, slightly radiating. 

 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 mark- 

 ings 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 striae and a well-defined pseudoraphe or median area. 



The genus "portant des stries en forme de boutonnieres," 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.) 



OPEPHORA SCHWARTZII (GRUN.) PETIT 



Valve obovate-lanceolate or nearly linear with rounded apices; striae transverse, broad, 

 3 or 4 in 10 p.; median area lanceolate. 



An inner stratum, with puncta in transverse rows, is apparent. 

 Blue clay. Not uncommon. Variable in size. 

 PL 10, Figs. 16 and 19. 



OPEPHORA PACIFICA (GRUN.) PETIT 



Valve linear, oblong, with rounded apices. Median area linear, narrow; striae punctate. 

 Blue clay. 

 PL 10, Fig. 18. 



Petit (Diat. Cap Horn) in his diagnosis states that the valves are cuneiform, but they 

 are not always so. 



