120 THE DIATOMACEffl OF PHILADELPHIA AND VICINITY 



time they move laterally to their extreme length and form a thread of frustules adherent 

 at their ends, later resuming their original position. The motion is repeated at intervals 

 of from five to ten seconds. No satisfactory explanation of the movement has ever been 

 made. In the filamentous form the frustules adhere to water-plants. 



VIVACES 



NITZSCHIA FLUMINENSIS GRUN. 



Valve lanceolate, apices produced; keel puncta, 4-6 in 10 ft, partly extended in short 

 costse; striae transverse, 14-15 in 10 n, punctate; keel without a pseudo-nodule. L. 73 n. 

 Common at Greenwich Point, Philadelphia. 

 PI. 32, Fig. 16. 

 The form here figured is smaller than the type, which is from 130-160 n in length. 



SPATHULATjE 

 NITZSCHIA SPATHULATA BREB. 



Frustule linear, truncate, dilated at the ends; zone with longitudinal folds; valve lance- 

 olate, keel central; apices acute, with an elevated appendage; keel puncta, 5-6 in 10 n; 

 striae very fine. L. 56 n. 



Atlantic City and Cape May, N. J. (Lewis). 



PI. 40, Fig. 3. 



DISSIPATE 



NITZSCHIA DISSIPATA (KUETZ.) GRTJN. 



Valve lanceolate, with sub-rostrate apices; keel excentric; keel puncta about 6 in 

 10 n; Btriae, 14 in 10 n. L. 20-40 M- 

 Fresh and brackish water. 

 PI. 40, Fig. 7. 



SIGMOIDE.E 



NITZSCHIA MACILENTA GREG. 



Frustule sigmoid, truncate at the ends; valve linear, with sub-acute apices and nearly 

 central keel; keel with 5-6 puncta in 10 n; striae obscure, about 25 to 28 (?) in 10 p. Length 

 variable, up to 490 M. 



As the valve is usually seen when the keel is on the margin, the outline (reduced one- 

 third, shown to the left of the figure) is, as a rule, sigmoid. 

 Delaware Bay. 

 PI. 33, Fig. 7. 



NITZSCHIA VERMICULARIS (KUETZ.) HANTZSCH 



Valve linear, sigmoid, attenuated toward the obtuse ends; keel puncta, 9 in 10 n, quite 

 distinct; striae very fine. L. 105 ju. 

 Fresh-water pools. 

 PI. 32, Fig. 24; PI. 33, Fig. 9. 



