. Fresh-water biology. Freshwater biology. 128 FRESH-WATER BIOLOGY 20(23,24,25) Central nodule small, rounded, or slightly elongated. ... 21 21(22) No lateral longitudinal areas of transverse septa. . NavictUa Bory. A form which grows in gelatinous tubes is regarded by some authors as a dififerent genus Schizonema but others regard it as a true Navicula. Fig. 77. Navicula rhynchocephala Kiitzing. X 557- (Original.) 22 (21) Two lateral longitudinal areas of transverse septa. Mostly imbedded in a gelatinous pseudothaUus. . Mastogloia Thwaites. In shape, Mastogloia resembles Navictda, but is dist
. Fresh-water biology. Freshwater biology. 128 FRESH-WATER BIOLOGY 20(23,24,25) Central nodule small, rounded, or slightly elongated. ... 21 21(22) No lateral longitudinal areas of transverse septa. . NavictUa Bory. A form which grows in gelatinous tubes is regarded by some authors as a dififerent genus Schizonema but others regard it as a true Navicula. Fig. 77. Navicula rhynchocephala Kiitzing. X 557- (Original.) 22 (21) Two lateral longitudinal areas of transverse septa. Mostly imbedded in a gelatinous pseudothaUus. . Mastogloia Thwaites. In shape, Mastogloia resembles Navictda, but is distinguished from it by the gelatinous envelope and the presence of lateral, transverse, siliceous septa or plates which divide the lateral regions of the cells into small compart- ments. There are transverse striations on the valves. It is not a very common genus in America. Fig. 78. Mastogloia smithii Thwaites. X about 300. (After Smith.) A B 23 (20, 24, 25) Central nodule broad, extending to near the margin of the valves. Stauroneis Ehrenberg. Siauroneis occurs frequently in all bodies of water and is a constituent of the diatomaceous flora which forms large siliceous deposits at the bottom of lakes. Fig. 79. Siauroneis anceps Ehrenberg. X 600. fOriginal.) 24 (20, 23, 25) Central nodule elongated to a short rod. Borne on gelatinous stalks Brebissonia Grun. Fig. 80. Brebissonia sp. X 580. (Original.) 25 (20, 23, 24) Central and end nodules elongated, enclosed with the raphe between two longitudinal, parallel, sihceous ribs. Frus- tules sometimes borne in gelatinous tubes. Vanheurchia Brebisson. Fig. 81. Vanlteurckia rhomhoides Ehrenberg. (After West.) X370. 26 (13) Valves asymmetrical with reference to the raphe or to a longitudinal axis; raphe arched, or nearer one margin than the other. Family Cymbellaceae .' . 27 27 (28) Valves greatly convex; girdle side elliptical or oval. Amphora Ehrenberg. ^ Van Heurck regards A mphora as one of the most difficult jjjjjjg genera of diatom
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