Pl. 21, Fig. 10. NEIDIUM PFITZER (1871) (neidion, dim. of naus, a boat) Valve linear or lanceolate; median fissures turned in opposite directions, terminal fissures appearing bifurcate (?); striæ transverse, usually oblique, finely punctate, crossed by one or several longitudinal blank lines. Chromatophores, two, lying on the girdle side, in cell division each forming a partially divided pair. A large pyrenoid is said to be found in the middle of each chromatophore, but Mereschkowsky states that the pyrenoids are absent, but that in N. affine four elæoplasts are always seen in the centre of the frustule. A genus easily recognized by the peculiar terminal and median fissures and by the yellowish or brownish color of the valves when dry, darker than in Caloneis. NEIDIUM AFFINE (EHR.) PFITZER Valve linear, with protracted, sub-rostrate or capitate ends. _Navicula affinis_ Ehr. NEIDIUM AFFINE VAR. GENUINA FORMA MAXIMA CL. Striæ, 14 in 10 µ, punctate, oblique in the middle, convergent at the ends; puncta, 15 in 10 µ. L. 238 µ. Pensauken, N. J. (artesian well). Pl. 21, Fig. 11. Var. genuina forma minor Cl.--L. 26 µ; striæ, 24 in 10 µ. Brandywine Creek. Pl. 21, Fig. 12. NEIDIUM AFFINE VAR. AMPHIRHYNCUS (EHR.) CL. Valve linear, with protracted capitate ends; striæ transverse, interrupted by several longitudinal lines. Willistown, Pa. Pl. 21, Fig. 13. NEIDIUM AMPHIGOMPHUS (EHR.) PFITZER Valve with parallel margins and cuneate ends; striæ transverse, interrupted by several longitudinal lines; central area widened transversely. _Navicula amphigomphus_ Ehr. Wissahickon. Pl. 21, Fig. 14. NEIDIUM PRODUCTUM (WM. SM.) CL. Valve linear, elongate, with capitate apices; striæ slightly oblique; longitudinal lines marginal; axial area very narrow, central area small. _Navicula producta_ Wm. Sm. Newtown Square. Pl. 21, Fig. 16. {84}NEIDIUM IRIDIS (EHR.) CL. Valve linear or lanceolate-elliptical, with sub-cuneate or rounded ends; striæ oblique, about 18 in 10 µ; central area orbicular. _Navicula iridis_ Ehr. _Navicula firma_ Kuetz. Willistown, Pa.; Middletown, Delaware Co., Pa. (Palmer). Pl. 21, Fig. 17. The form here figured is probably the variety ampliata (Ehr.) Cl. with less acute apices and more elliptical outline. The species occurs in many variations, the larger being found northward, especially in the peat deposits of New England. NEIDIUM HITCHCOCKII (EHR.) CL. Valve linear, with triundulate margin and cuneate ends; striæ transverse, oblique. _Navicula hitchcockii_ Ehr. Pavonia, N. J. (artesian well); Kirkwood Pond, N. J. Pl. 21, Fig. 15. DIPLONEIS EHR. (1840) (diplos, double)