Forma parva V. H.—Keel puncta, 10-12 in 10 µ. L. 70 µ.
East Park Reservoir, Philadelphia.
Pl. [33], Fig. 10.
Differs from N. closterium (Ehr.) Wm. Sm. in the keel puncta.
The type form occurs in brackish and salt water. The occurrence of the variety in fresh water is another instance of the finding of presumably brackish forms in the water supply of the city. If these cases prove to be unusual, it may be because of one of two reasons. The Schuylkill River, before the building of the dam at Fairmount, was tidal as far as the Falls of Schuylkill, and brackish influences, while not now existent, may have caused the growth of forms which now survive. Another reason may be that the opening of the locks at Fairmount Dam may cause a slight admission of brackish forms from tidal water below. The abundance of the brackish species appears to indicate that the first reason is the more plausible.
NITZSCHIA REVERSA WM. SM.
Valve lanceolate extended into beaks or horns curving in opposite directions; keel puncta not evident; striæ, "20-26" in 10 µ. L. 70 µ.
Brackish water. Abundant in Duck Creek, Delaware River.
Pl. [33], Fig. 11.
NITZSCHIA ACICULARIS (KUETZ.) WM. SM.