Valve linear, sigmoid, attenuated toward the obtuse ends; keel puncta, 9 in 10 µ, quite distinct; striæ very fine. L. 105 µ. Fresh-water pools. Pl. 32, Fig. 24; Pl. 33, Fig. 9. {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 µ; striæ, 20-24 in 10 µ. L. to 250 µ. Along the coast. Pl. 39, Fig. 13. NITZSCHIA SIGMATELLA GREG. Valve linear, sigmoid, slightly attenuated toward the obtuse apices; keel excentric, puncta, 8-10 (?) in 10 µ; striæ delicate, 25-30 in 10 µ. L. to 400 µ. 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 striæ in 10 µ, 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 µ; striæ subtle. L. 40 µ. Abundant in Ridley Creek, Delaware Co. (Palmer). Pl. 32, Fig. 20. OBTUSÆ 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 µ; striæ, 26 in 10 µ. L. to 300 µ. Along the coast. Pl. 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 µ; striæ, 26 in 10 µ. L. 48 µ. Along the coast. Pl. 32, Fig. 17. {122}SPECTABILES NITZSCHIA SPECTABILIS VAR. AMERICANA GRUN. Frustule linear, slightly constricted in the middle, with sub-cuneate ends; valve linear, slightly arcuate, tapering to the sub-rostrate ends; keel excentric, keel puncta sometimes confluent, 4-6 in 10 µ, prolonged into short costæ; striæ distinct, 14 in the middle, 18 at the ends in 10 µ (but variable in different specimens). L. 186 µ. Blue clay, especially at Tioga St. Pl. 33, Fig. 3; Pl. 39, Fig. 1. This is, probably, one of the most beautiful of the Nitzschiæ. It sometimes, according to De Toni, reaches a length of 520 µ. Grunow states that his variety is found in the S. Bridgeton deposit. In a slide of Moeller labelled "Bridgeton, Maine," I find specimens identical in every respect with the Philadelphia form. LINEARES NITZSCHIA LINEARIS (AG.) WM. SM. Valve linear, slightly inflexed in the middle; keel excentric; keel puncta, 8-9 in 10 µ, the two median distant; striæ about 30 in 10 µ. Frustules in zone view narrowed toward the ends, truncate. L. 75 µ. Very common in fresh water.