DIMEROGRAMMA MARINUM (GREG.) RALFS Valve lanceolate or linear and inflated in the middle; striæ moniliform, transverse or slightly radiate; median area linear or lanceolate, sometimes not reaching the smooth extremities; striæ, 8 in 10 µ. Pl. 12, Figs. 9 and 10. Fig. 9 differs in its lanceolate outline, in having four puncta on each side in a row, and in the striæ which are radiate. DIMEROGRAMMA SURIRELLA (EHR.) GRUN. Valve elliptical-lanceolate, with rounded apices; striæ moniliform, radiate; pseudoraphe narrow, lanceolate. Blue clay. Pl. 12, Fig. 11. {47}DIMEROGRAMMA MINUS (GREG.) RALFS Valve rhombic-lanceolate; striæ punctate, radiate; pseudoraphe lanceolate; apices smooth. Blue clay. Along the coast. Pl. 12, Figs. 12, 13, 14. TRACHYSPHENIA PETIT (1877) (trachys, rough, and sphen, a wedge) Frustules rectangular. Valve cuneiform with coarse puncta in transverse and longitudinal lines; pseudoraphe narrow, linear. One species only. TRACHYSPHENIA AUSTRALIS PETIT Characters of the genus. Valve small; puncta, 6 in 10 µ. Allied to Dimerogramma. Shark River, N. J. Rare. Pl. 12, Fig. 15. SYNEDRA EHR. (1830) (synedrion, a sitting together) Frustules adnate in small stipitate clusters or free. Valve elongate, linear or linear-lanceolate; pseudoraphe distinct; costæ absent. The genus Synedra has few distinctive characters. As Brun remarks (Diat. des Alpes et du Jura, p. 122), the dilatation of the extremities and the pseudo-nodule are of little value in classification, as the intermediate forms are so numerous. Fragilaria occurs in very long ribbons or fasciæ, Synedra in short fasciæ or radiating clusters. Fragilaria is seldom longer than three or four times the width, while Synedra is nearly always so. The former has fine, often subtle, markings and narrow pseudoraphe, while the latter has coarser punctate striæ and a more distinct pseudoraphe. Chromatophores usually consist of two bands, one on each of the valves. Karsten states that in the marine forms the chromatophores are oval or polygonal discs, each of which usually encloses a pyrenoid. SYNEDRA ULNA (NITZSCH) EHR. Frustules solitary or in twos. Valve 150-250 µ in length, linear or linear-lanceolate, with rostrate apices; striæ, 9 in 10 µ. Common in rivers and streams. Pl. 11, Figs. 4, 7 and 11 (?). Frequently interrupted in the middle. The distinction made by Wm. Smith as to the presence or absence of the central blank space is probably not necessary, as both forms are found which are otherwise identical. Fig. 5 represents the formation of a sporangial frustule which differs from the usual form in its inflated ends prolonged into rostrate apices. Figs. 1 and 6 are sporangial frustules. {48}SYNEDRA BICEPS (KUETZ.) SCHMIDT Valve sublanceolate, inflated at the ends, apices rounded; central space not always distinct; pseudoraphe narrow; striæ radiate at the ends.