pyrenoids; cells 3-6^ in diameter; colony 7-15/i, wide without setae, 

 which are 4-8/1, long. 



This species should be compared with Crucigenia quadrata. 



Rare in the plankton of several lakes. Mich., Wis. 



TETRALLANTOS Teiling 1916, p. 62 



A colony of 4 crescent-shaped or sausage-shaped cells which are 

 bluntly rounded at their apices and inclosed by a colonial mucilage, 

 with clusters of daughter cells often held in approximation by old 

 mother cell wall fragments; cells in 2 pairs and in 2 planes, 1 pair 

 facing each other and in contact at their poles, the other pair in a 

 longitudinal plane vertical to these and so arranged that each 

 member has 1 pole at the point of contact of the poles of the other 

 pair; chloroplast a parietal plate with 1 pyrenoid. 



Tetrallantos Lagerheimii Teiling 1916, p. 62 

 PL 66, Figs. 4-6 



Characteristics as described for the genus; cells 4-8/x, in diameter, 

 lO-lSfx long. 



This rather unusual plant is rare in the United States, apparently 

 having been found only in Wisconsin, Massachusetts, and Mississippi. 



Rare; in the plankton of soft water lakes and small ponds. Wis. 



MICRACTINIUM Fresenius 1858, p. 236 



Free-floating, one-celled or in colonies of tetrahedrally or pyramid- 

 ally arranged spherical or ovate cells (sometimes in 1 plane); walls 

 beset with 1-7 long, needle-like setae; 1 parietal, cup-shaped 

 chloroplast containing a single pyrenoid. Korschikov (1937) reports 

 observation of sexual reproduction in this genus. 



Key to the Species 



Cells spherical; setae 20-35/x long M. pusillum 



Cells ovate; setae 23-40^ long___ M. quadrisetum 



Micractinium ptisillum Fresenius 1858, p. 236 

 PI. 66, Fig. 8 



A free-floating colony of 4-16 spherical cells arranged in a 

 pyramid or in a square, groups of 4 in association with other 

 similar groups; free walls beset with 1-5 finely tapering setae; 

 chloroplast a parietal cup with 1 pyrenoid; cells S-l/x in diameter 

 without setae; setae 20-35ja long. 



Rare in the plankton of many lakes, mostly soft water habitats. 

 Mich., Wis. 



[287] 



