132 



FRESH-WATER BIOLOGY 



5 2 (5 1 , 53) Characteristics similar to those of Diatoma except that the cells 

 are borne in ribbons Denticula Kiitzing. 



The valves are marked by heavy ribs which are in reality shallow 

 septa, between which are delicate striae. 



Denticula occurs on wet rocks and in fresh water; sometimes also in 

 brackish water. 



FIG. 96. Denticula inflata Smith, a. Valve side. b. girdle side. 

 X6oo. (Original.) 



53 (5 1 ) 5 2 ) Characteristics as in Denticula except that the striations are in- 

 terrupted in the middle Odontidium Kiitzing. 



Many place the members of this genus with Diatoma, while 

 others regard the interrupted striae and the formation of short fila- 

 ments instead of zig-zag chains, sufficient differences to place them 

 in a separate genus. 



FIG. 97. a, b. Odontidium mutabile Smith, c. Odontidium tabellaria 

 Smith. X 570. (Original.) 



54 (50) Valves asymmetrical with reference to a transverse axis. 



Family MERIDIONACEAE. 

 Only one genus Meridian Agardh. 



Both valve and girdle sides wedge-shaped, forming ring- 

 like or fan-shaped bands; striations uninterrupted. 



There are imperfect transverse septa which are con- 

 spicuous on the valve side but show only laterally on 

 the girdle side. Between these on the valve side are fine 

 punctate striae. 



Van Heurck thinks this genus ought to be suppressed. 

 It differs from Diatoma only in the cuneate shape of the 

 valves. 



FIG. 98. 



Meridian constrictum Ralfs. 

 Smith.) 



X 300. (After 



55 (49) Valves without transverse costae. . Family FRAGILARIACEAE . . 56 



Cells of much the same structure as Diatoma. Transverse striations composed of separate 

 dots; with or without raphe and end nodules. 



5 6 (57> 5 8 ) Cells very slender, not united in bands, either free or attached at 



one end, forming clusters on higher algae. 



Synedra Ehrenberg. 



57 (S^, 5 8 ) Cells forming bands or zig-zag chains. . . Fragilaria Lyngbye. 



Fragilaria is a common genus oc- 

 curring in ponds, reservoirs, and lakes. 

 F. crotonensis has been known to occur 

 in such quantities as to form water 

 bloom, producing a thick brown scum 

 on the surface of a lake. 



FIG. ioo. Fragilaria crotonensis Kitton. 

 a. Valve side. b. girdle side. X 225. 

 (Original.) 



