Pl. 32, Fig. 18. Fig. 20, Pl. 40, a transverse section of frustule. LANCEOLATÆ NITZSCHIA PALEA (KUETZ.) WM. SM. Valve linear-lanceolate, slightly rostrate at the apices; keel puncta, 10 in 10 µ, the median not distant; striæ, 33-36 in 10 µ; zone view linear, with rounded ends. L. 25-65 µ. Fresh water. Pl. 32, Fig. 15. NITZSCHIA AMPHIBIA GRUN. Valve lanceolate, apices sometimes slightly produced, rounded; keel puncta, 8-9 in 10 µ; striæ, 16 in 10 µ. L. 20-32 µ. Fresh water. Pl. 32, Figs. 14 and 25. NITZSCHIA COMMUNIS RAB. Frustule linear, slightly attenuated at the obtuse ends; valve elliptical-lanceolate, attenuated toward the obtuse ends; keel puncta, 12 in 10 µ; striæ more than 30 in 10 µ. L. 35 µ. Fresh water. Pl. 32, Fig. 19. NITZSCHIA INTERMEDIA HANTZSCH Valve linear-lanceolate; keel puncta, 8 in 10 µ; striæ about 24 in 10 µ. L. 100 µ. Crum Creek. Not common. Pl. 33, Fig. 2. {123}NITZSCHIELLA NITZSCHIA LONGISSIMA (BRÉB.) RALFS Valve linear-lanceolate, with exceedingly long horns or beaks; keel puncta about 10 in 10 µ; striæ about 16 in 10 µ. L. to 500 µ. Shark River, N. J. Pl. 33, Fig. 1. 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. Valve lanceolate, with beaks or horns about half the length of the median part of the valve; keel puncta, 18 in 10 µ; striæ exceedingly delicate, "about 40 in 10 µ." L. 45 µ. Fresh water. Darby Creek. Pl. 33, Fig. 12. HOMOEOCLADIA AG. (1827) (homoios, like, and clados, a branch) Frustules like Nitzschia, but enclosed in branching or simple tubes. HOMOEOCLADIA FILIFORMIS WM. SM. Frustule linear, tumid in the middle, obtuse at the ends; valve linear-lanceolate, with somewhat acute apices; keel central or nearly so; keel puncta, 8 in 10 µ; striæ delicate. L. 108 µ. Fresh and brackish water. Newark, N. J. Pl. 33, Fig. 15. {124}SURIRELLA TURPIN (1828)