There is difficulty in recognizing S. radians K. as described and figured by different authors. On Plate 12, Fig. 8, I have drawn a specimen from H. L. Smith's Type Slide No. 574, labelled S. radians Kuetz., not Wm. Smith, which, however, corresponds closely to Smith's figure (Brit. Diat. 1, Pl. 11, Fig. 89). De Toni gives S. radians Kuetz. as equivalent to S. tenera Wm. Sm. Van Heurck's figure of S. radians, and also the figure of ulna var., said to be synonymous with H. L. Smith's S. radians, which does not correspond to the specimens on Smith's slide in my possession, are confusing. In Van Heurck's Synopsis the striæ are said to be 16 or 17, while De Toni describes them as subtle and from 17 to 24 in 10 µ. The length is quite variable. Several species of Synedra resemble S. radians in the mode of growth, as they are adnate at first, in short bands, the frustules being sessile on other plants or objects, attached at the terminal nodules which, although scarcely visible in most forms, are probably present in all. The frustules are not closely connected at the free end, and soon become entirely detached. In Diatoma and Fragilaria, we find a punctum or pore at one end of a valve, but not in line with the pseudoraphe; in Synedra, a minute pore is usually found in the position of the terminal nodule and, in some species, indications of a central nodule are observed; the median line is wider but there is no raphe. In the fresh-water Synedræ, many of which are among the longest of diatoms, living in running streams, the terminal nodules are much more indistinct, while the marine forms have distinct terminal nodules, are not, as a rule, found in bands, and assume a more naviculoid outline. SYNEDRA VAUCHERIÆ VAR. PARVULA (KUETZ.) RAB. Valve lanceolate, with produced or rostrate apices; pseudo-nodule wide, excentric. L. 17 µ. Crum Creek. Pl. 12, Fig. 5. Fig. 6 represents a variety with coarser striæ from the Schuylkill River. Both are easily mistaken for Fragilaria intermedia. {50}SYNEDRA FULGENS (GREV.) WM. SM. Frustules geminate or flabellate on a stipe. Valve slightly inflated in the middle and at the apices; pseudoraphe narrow; striæ finely punctate, radiate at the ends. Marine. Atlantic City. Pl. 11, Fig. 10. SYNEDRA AFFINIS KUETZ. Valve lanceolate; striæ marginal, leaving a broad lanceolate pseudoraphe. Common along the coast. Pl. 12, Fig. 3. SYNEDRA AFFINIS VAR. PARVA (KUETZ.) V. H. Valve lanceolate, slender; striæ marginal, shorter than in the type. _Synedra gracilis_ Kuetz. Common along the coast. Pl. 12, Fig. 7. SYNEDRA AFFINIS VAR. TABULATA (AG.) V. H. Valve linear-lanceolate; striæ, 11 in 10 µ, very short. Not common. New Rochelle. Pl. 12, Fig. 4. ASTERIONELLA HASSALL (1855) (dim. of aster, a star) Frustules linear, slightly inflated at the ends, arranged in star-shaped clusters which soon break up. Valve linear, unequally inflated at the ends. ASTERIONELLA FORMOSA HASS.