44 THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 



OPEPHORA PINNATA VAR. LANCEOLATA N. VAR. 



Valve lanceolate; costse slightly radiate, punctate; median area broad, lanceolate. 

 Differs from 0. pinnata in outline, radiation of the costse and median area. 

 Blue clay. Rare. 

 PL 10, Fig. 17. 



FRAGILARIINjE 



Fragilaria. Frustules in fasciae. Valve with transverse striae. Pseudoraphe indistinct. 



Rhaphoneis. Striae radiate; pseudoraphe distinct. 



Dimerogramma. Pseudoraphe broad. 



Trachysphenia. Valve cuneiform. 



Synedra. Valve elongate. 



Asterionella. Frustules in star-shaped clusters. 



FRAGILARIA (LYNG.) RAB. 

 (fragilis, because of the fasciae easily breaking up) 



Frustules rectangular, adnate in fasciae, soon breaking up. Valve lanceolate, oblong 

 or elliptical in general outline, with convex or sinuate margins; without costse; pseudoraphe 

 narrow or indistinct; striae 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 fasciae. Valve elliptical-lanceolate, obtuse at the apices; pseudoraphe 

 indistinct; striae, 17 in 10 ju, punctate. 



Very common in springs and pure streams. The fasciae are often a foot or more in 

 length. 



PI. 10, Figs. 20 and 21. 



FRAGILARIA ARCTICA GRUN. 



Valve oblong or elliptical, 10 ^ in length; striae subtle, with coarse, short striae at inter- 

 vals on the margin and evident in zone view. 

 Marine. Common at Cape May, N. J. 

 PI. 10, Figs. 22 and 23. 



FRAGILARIA UNDATA WM. SM. 



Valve in general outline linear-elliptical, with extremities produced; striae sub tie; pseudo- 

 raphe distinct. 

 Fresh water. 

 PI. 10, Figs. 24, 25, 27, 28 and 29. 



