132 



FRESH-WATER BIOLOGY 



52 (51, 53) Characteristics similar to those of Dialoma except that the cells 

 are borne in ribbons. . Denticula KUtzing. 



The vajves are marked by heavy ribs which are in reaUty shallow 

 septa, between which are delicate striae, 

 i iiii iiii ' t Denticula occurs on wet rocks and in fresh water; sometimes also in 



""""""I brackish water. 



Fig. 96. Denticula injlata Smith, a. Valve side. 6. girdle side. 

 X 600. (Original.) 



S3 (Si; 52) Characteristics as in Denticula except that the striations are in- 

 terrupted in the middle Odontidium KUtzing. 



Many place the members of this genus with Dialoma, while 

 others regard the interrupted striae and the formation of shoirt 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 tabetlaria 

 Smith. X 570. (Original.) 



S4 (so) 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 Dialoma only in the cuneate shape of the 

 valves. 



Fig. 98. 



Meridion constriclum Ralfs. X 300. 

 Smith.) 



(After 



SS (49) Valves without transverse costae. . Family Fragilaeiaceae . . 56 



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

 dots; with or without raphe and end nodules. 



56 (S7, 58) Cells very slender, not united in bands, either free or attached at 

 one end, forming clusters on higher algae. 



Synedra Ehrenberg. 



Fig. 99. Synedra salina W. Smith. 

 X 588. (Original.) 



Fragilaria Lyngbye. 



Sfi 



S7 (56, 58) Cells forming bands or zig-zag chains. 



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. 100. Fragilaria crotonensis Kittoa 

 D fl. Valve side. h. girdle side. X 22'" 

 * (Original.) 



