Delaware Bay. Pl. 33, Fig. 8. BACILLARIA NITZSCHIA PAXILLIFER (O. F. MUELLER) HEIBERG Frustules united in a filament, afterwards free; valve lanceolate with nearly central keel; keel puncta, 7-9 in 10 µ; striæ about 21 in 10 µ. L. 110 µ. _Vibrio paxillifer_ O. F. Mueller. _Bacillaria paradoxa_ Gmelin. _Nitzschia paradoxa_ (Gmelin) Grun. Brackish water or streams subject to its influence. Pl. 33, Figs. 13 and 14. Otto Frederick Mueller, in 1786, published at Copenhagen a work on "Infusorial Animalcules," including a description of a Vibrio which he named paxillifer, obviously alluding to the partially-extended frustules bearing at the end a tablet-like bundle. Two years later, Gmelin described the same form as Bacillaria paradoxa, a name still used. Heiberg, however, in 1863, placed the form under Nitzschia where it properly belongs and called it Nitzschia paxillifer (O. F. Mueller). I have adopted Heiberg's name. Perhaps the most remarkable of all diatoms. Many species possess the power of motion, which, however, is evident only in the free frustule. In N. paxillifer, the movement of the frustules occurs without the loss of continuity or adherence to each other, so that, while at one time the adnate frustules form a narrow filament, like that of Fragilaria, at another {120}time they move laterally to their extreme length and form a thread of frustules adherent at their ends, later resuming their original position. The motion is repeated at intervals of from five to ten seconds. No satisfactory explanation of the movement has ever been made. In the filamentous form the frustules adhere to water-plants. VIVACES NITZSCHIA FLUMINENSIS GRUN. Valve lanceolate, apices produced; keel puncta, 4-6 in 10 µ, partly extended in short costæ; striæ transverse, 14-15 in 10 µ, punctate; keel without a pseudo-nodule. L. 73 µ. Common at Greenwich Point, Philadelphia. Pl. 32, Fig. 16. The form here figured is smaller than the type, which is from 130-160 µ in length. SPATHULATÆ NITZSCHIA SPATHULATA BRÉB. Frustule linear, truncate, dilated at the ends; zone with longitudinal folds; valve lanceolate, keel central; apices acute, with an elevated appendage; keel puncta, 5-6 in 10 µ; striæ very fine. L. 56 µ. Atlantic City and Cape May, N. J. (Lewis). Pl. 40, Fig. 3. DISSIPATÆ NITZSCHIA DISSIPATA (KUETZ.) GRUN. Valve lanceolate, with sub-rostrate apices; keel excentric; keel puncta about 6 in 10 µ; striæ, 14 in 10 µ. L. 20-40 µ. Fresh and brackish water. Pl. 40, Fig. 7. SIGMOIDEÆ NITZSCHIA MACILENTA GREG. Frustule sigmoid, truncate at the ends; valve linear, with sub-acute apices and nearly central keel; keel with 5-6 puncta in 10 µ; striæ obscure, about 25 to 28 (?) in 10 µ. Length variable, up to 490 µ. As the valve is usually seen when the keel is on the margin, the outline (reduced one-third, shown to the left of the figure) is, as a rule, sigmoid. Delaware Bay. Pl. 33, Fig. 7. NITZSCHIA VERMICULARIS (KUETZ.) HANTZSCH