60 THE DIATOMACE^E OF PHILADELPHIA AND VICINITY 



This form I found in a pure gathering covering the sides of a greenhouse tank at Elm, 

 N. J. It was sent to Prof. H. L. Smith, who determined it as forma curta of A. linearis. 

 PI. 16, Figs. 16 and 17. 



ACHNANTHES DANICA (PLOEGEL) GRUN. 



Valves rhombic-lanceolate, with subacute ends. Striae, 25 in 10 p., radiate. Lower 

 valve with stauros widened toward the margin, and cleft into three divisions. 



Pavonia, N. J. (artesian well). 



PI. 16, Fig. 13. 



I have seen the lower valve only. Cleve states that the upper valve is costate with 

 "alternating fine lineolae twice as close as the costse." 



CYMBELLA AG. 

 (cymbe, a boat) 



Frustules free, stipitate or enclosed in tubes. Valve boat-shaped; median line asym- 

 metrical, straight or curved. 



Chromatophore single, covering the entire ulterior of the frustule, except the ventral 

 part of the zone and the median lines. Its longitudinal axis is on the dorsal part of the zone. 

 A pyrenoid lies in a fold of the chromatophore on the dorsal part. 



The genus includes the former genera of Cocconema, characterized by stipitate forms, 

 and Encyonema in which the frustules are frequently enclosed in gelatinous tubes. 



SECTION 1. CYMBELLA PROPER. FRUSTULES FREE OR SOMETIMES STIPITATE 



CYMBELLA HETEROPLEURA (EHR.) KUETZ. 



Valve nearly symmetrical, lanceolate, with rostrate, produced apices; median line 

 nearly straight; axial area linear, widened in the middle; striae radiate, punctate. 

 Blue clay. 

 PL 18, Fig. 10. 



CYMBELLA CUSPIDATA KUETZ. 



Valve broad, elliptical, with rostrate, somewhat acute, apices and nearly straight, ven- 

 tral margin; median line straight, axial area linear, widened in the middle; strise radiate, 

 punctate. 



Blue clay. 



PI. 18, Fig. 17. 



CYMBELLA NAVICULIFORMIS AUERSWALD 



Valve linear-elliptical, with abruptly produced apices: ventral margin straight; median 

 line almost straight; axial area narrow, central area large, rounded; striae distant in the 

 middle, closer at the ends. 



Fresh water. 



PI. 18, Fig. 6. 



CYMBELLA EHRENBERGII KUETZ. 



Valve lanceolate, with ventral margin nearly straight and apices sub-rostrate; median 

 line straight, excentric; axial area narrow; central area widened in the middle; striae coarsely 

 punctate. 



Fresh water. 



PI. 18, Fig. 9. 



