Pl. 16, Figs. 4, 5, 6. The three species described above are named from the length of the stipe, but this varies considerably and is not of special significance. ACHNANTHES INFLATA (KUETZ.) GRUN. Valves more or less inflated in the middle, usually with the stauros of the lower valve asymmetrical and wider than in A. subsessilis, with which it agrees in size and markings. Gloucester, N. J. (artesian well). Pl. 16, Figs. 7 and 8. ACHNANTHES COARCTATA (BRÉB.) GRUN. Valves lanceolate, oblong, broad at the ends and constricted in the middle. Stauros wide; pseudoraphe of the upper valve excentric; striæ slightly radiate on the lower valve; puncta small. Blue clay. Pl. 16, Fig. 9. ACHNANTHES LANCEOLATA (BRÉB.) GRUN. Valves more or less elliptical; striæ radiating, 12 in 10 µ, punctate; on the lower valve a horse-shoe shaped hyaline space on one side of the centre; on the upper valve an irregular stauros, not reaching the margin. L. 8-20 µ. In springs. Abundant at Newtown Square. Pl. 16, Figs. 10, 11, 12. ACHNANTHES EXIGUA GRUN. Valves oblong-lanceolate, with rostrate ends, sometimes slightly constricted in the middle. Stauros rather wide; striæ punctate, radiating, 22 in 10 µ. L. 10-12 µ. _Stauroneis exilis_ Kuetz. (not Achnanthes exilis Kuetz.) Frequently found in aquaria where I have kept it growing continuously for years. Pl. 16, Figs. 14 and 15. ACHNANTHES LINEARIS FORMA CURTA H. L. SMITH Frustules solitary or geminate. Valves linear-elliptical, or elliptical-lanceolate. Lower valve without distinct axial area; upper valve with axial area widened in the middle; striæ slightly radiate (?). L. 7 µ. One of the smallest of diatoms. {60}This form I found in a pure gathering covering the sides of a greenhouse tank at Elm, N. J. It was sent to Prof. H. L. Smith, who determined it as forma curta of A. linearis. Pl. 16, Figs. 16 and 17. ACHNANTHES DANICA (FLOEGEL) GRUN. Valves rhombic-lanceolate, with subacute ends. Striæ, 25 in 10 µ, radiate. Lower valve with stauros widened toward the margin, and cleft into three divisions. Pavonia, N. J. (artesian well). Pl. 16, Fig. 13. I have seen the lower valve only. Cleve states that the upper valve is costate with "alternating fine lineolæ twice as close as the costæ." CYMBELLA AG. (1830) (cymbe, a boat) Frustules free, stipitate or enclosed in tubes. Valve boat-shaped; median line asymmetrical, straight or curved. Chromatophore single, covering the entire interior of the frustule, except the ventral part of the zone and the median lines. Its longitudinal axis is on the dorsal part of the zone. A pyrenoid lies in a fold of the chromatophore on the dorsal part. The genus includes the former genera of Cocconema, characterized by stipitate forms, and Encyonema in which the frustules are frequently enclosed in gelatinous tubes. SECTION 1.--CYMBELLA PROPER. FRUSTULES FREE OR SOMETIMES STIPITATE CYMBELLA HETEROPLEURA (EHR.) KUETZ.