shallow water of swamps and marshes. Its rapid rate of reproduction 

 (daughter nets formed within each cell of the parent net) makes it 

 possible for Hydrodictyon to develop luxuriant growths in favorable 

 habitats. Thick floating mats often result, and sometimes unbal- 

 anced biological conditions are produced. In some sections it 

 becomes an obnoxious weed, clogging filters, drains, etc. It is so 

 definitely confined to hard water tJhiat it may be used as an index 

 organism for a high pH. In the far West it finds ideal growing 

 conditions in the alkahne water of irrigation reservoirs and ditches. 

 Common in a large number of lakes, mostly hard water; generally 

 distributed. Mich., Wis. 



EUASTROPSIS Lagerheim 1895, p. 20 

 Thallus free-floating, composed of 2 flattened, triangular or 

 tiapezoidal cells adjoined along one wall, the lateral free margins 

 converging and slightly concave, the apex deeply notched; chloro- 

 plast 1, parietal, with 1 pyrenoid. 



Euastropsis Richteri (Schmidle) Lagerheim 1895, p. 20 



PL 47, Fig. 2 

 Characters as described for the genus; cells 4.5-25)U, in diameter, 

 5-20/A long; 2-celled colony 4.5-25/x wide, 10-40/x long. 

 Rare, in plankton. Wis. 



PEDIASTRUM Meyen 1829, p. 772 

 Coenobium a free-floating, circular, monostromatic disc of cells 

 which may be continuous or perforate; peripheral cells of the disc 

 with 1 or 2 lobes or processes, or merely emarginate without 

 processes; interior cells either the same shape as the marginal ones 

 or different; chloroplast a parietal reticulum, covering the wall, 

 with 1 pyrenoid; cells multinucleate. 



Because the plates of Pediastrum are formed by the juxaposition 

 of zoospores developed within a vesicle which is extruded from the 

 mother cell, it not infrequently happens that irregularly formed or 

 abnormal coenobia develop when the zoospores fail to align them- 

 selves in one plane. Hence coenobia may be found in which some 

 cells overlie others. For a criticial study of this genus the reader is 

 referred to Bigeard (1933) and Harper (1916, 1918). 



Key to the Species 



1. Outer free wall of the peripheral cells extended to form 



a single, horn-like projection P. simplex 



1. Outer free wall of peripheral cells with 2 to 4 projections, or 



merely emarginate, not forming processes 2 



[ 220 ] 



