indistinct in most species and confluent with the colonial mucilage. 

 The plate-like form of the Merismopedia colony and the regularity 

 of cell arrangement make identification of this genus certain. Only 

 in rare instances are Chroococcus cells found in a somewhat plate- 

 like colony, and the examination of a number of individuals will 

 confirm the identification. 



Key to the Species 



1. Colony a very broad, membranaceous plate with 



margins convolute and enrolled M. convoluta 



1. Colony a flat plate 2 



2. Cells with refractive vacuoles - M. Trolleri 



2. Cells without refractive vacuoles, usually homogeneous and 



finely granular, sometimes with a few conspicuous granulations 3 



3. Cells hght blue-green, finely granular _— 4 



3. Cells bright blue-green, frequently with coarse granulations M. elegans 



4. Cells forming small rectangular colonies; 



1.3-2. 2^1 in diameter M. tenuissima 



4. Cells larger - -.- 5 



5. Cells 3-7yu in diameter M. glauca 



5. Cells averaging smaller than above - 6 



6. Cells blue-green or gray-green, 2.5^ju. 



in diameter; sheaths thin. M. punctata 



6. Cells violet or somewhat pink, inclosed by 



thick, cartilaginous sheaths... M. chondroidea 



Merismopedia chondroidea Wittrock & Nordstedt 1878, No. 200 



A colony of 4-8 (rarely 16) cells with thick individual sheaths, 

 arranged in a rectangular plate; cell contents violet or somewhat 

 pink; cells 2-3. 5/* in diameter. 



Geitler (1928) refers to this plant as an abnormal expression of 

 some other species. It has appeared too seldom in our collections 

 to provide a satisfactory basis for a final decision, but I use the 

 name for forms of Merismopedia that have thick sheaths. 



In the tychoplankton of several soft water lakes and Sphagnum 

 bogs. Wis. 



Merismopedia convoluta de Brebisson in Kuetzing 1849, p. 472 



PI. 103, Fig. 13 



Colony irregularly quadrangular, forming extensive sheets with 

 convolute margins, 1-4 mm. in width and usually visible to the 

 unaided eye; cells spherical or oblong, 4-5|U. in diameter, 4-8/a long, 

 arranged in multiple families of 64 individuals. 



Among other algae in tychoplankton; in a dense film of miscel- 

 laneous algae on submerged wood. Mich., Wis. 



[458] 



