THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 121 



SIGMATA 

 NITZSCHIA SIGMA (KUETZ.) WM. SM. 



Frustule linear, sigmoid; valve linear, slightly sigmoid, tapering to the sub-acute 

 apices; keel excentric, puncta, 8 in 10 p.; striae, 20-24 in 10 n. L. to 250 p. 

 Along the coast. 

 PI. 39, Fig. 13. 



NITZSCHIA SIGMATELLA GREG. 



Valve linear, sigmoid, slightly attenuated toward the obtuse apices; keel excentric, 

 puncta, 8-10 (?) in 10 M; striae delicate, 25-30 hi 10 /i. L. to 400 p. The keel puncta are 

 quite obscure. 



Nitzschia curvula Wm. Sm. 



Nitzschia sigma var curvula (Wm. Sm.) De Toni. 



Fresh water. Hammonton Pond; May's Landing, N. J. 



PL 33, Figs. 4 and 5. 



Gregory remarks that the keel puncta are seen in some specimens. In both of the forms 

 figured I have counted 30 striae in 10 n, but, after many examinations, I have not been 

 quite certain about the keel puncta. The general appearance of the valves in any position 

 is that of a Stenopterobia or Surirella anceps, with which it occurs. 



NITZSCHIA CLAUSII HANTZSCH 



Valve linear, slightly sigmoid, tapering to the sub-capitate ends; keel puncta, 11 in 

 10 n; striae subtle. L. 40 ju- 



Abundant in Ridley Creek, Delaware Co. (Palmer). 

 PL 32, Fig. 20. 



OBTUSE 



NITZSCHIA OBTUSA WM. SM. 



Frustule sigmoid, rounded at the ends; keel somewhat excentric, inflexed in the middle, 

 the two median puncta distant; keel puncta, 5-6 in 10 M; striae, 26 in 10 M- L- to 300 p. 

 Along the coast. 

 PI. 39, Fig. 16. 



NITZSCHIA OBTUSA VAR. FLEXELLA H. L. SMITH 



Valve more attenuate at the ends than the type and smaller. 

 PL 39, Fig. 14. 



NITZSCHIA OBTUSA VAR. SCALPELLIFORMIS GRUN. 



Valve linear, with apices unilaterally truncate; keel excentric; keel puncta, 8 in 10 /*; 

 striae, 26 in 10 M- L- 48 p. 

 Along the coast. 

 PL 32,. Fig. 17. 



