and free surface of the cell furnished with 1-4 stout, outwardly 

 directed spines; 1 diffuse chloroplast covering most of the wall, 

 containing a single pyrenoid. 



Key to the Species 



Cells broadly cuneate; spines 4-8/i long S. spinulosum 



Cells narrowly cuneate; spines lO-lS^a long S. americanum 



Sorastrum americanum (Bohlin) Schmidle 1900d, p. 230 



PL 50, Fig. 8 



A free-floating spherical colony of 16-128 heart-shaped or sub- 

 pyramidate cells with the outer free walls emarginate and furnished 

 at each of the 4 angles with a long, stout, outwardly directed spine; 

 cells narrowed toward the base and attached to the center of the 

 colony by a short, cylindrical stalk which at the base is 5- or 6-sided 

 adjoining the sides of other stalks in such a way as to form a central 

 hollow sphere; cells 7-20)u, in diameter, 5-20ju, long, 4-8/a thick; 

 spines 10-15yu, long. 



Rare; in several soft water lakes, especially in northern counties; 

 eu- and tychoplanktonic. Mich., Wis. 



Sorastrum americanum, var. undulatum G. M. Smith 1918, p. 640 



PI. 50, Fig. 10 

 A variety differing from the typical in having the margins of the 

 facets at the base of the stipe undulate. 

 Rare; in the euplankton of lakes. Wis. 



Sorastrum spinulosum Naegeli 1849, p. 99 

 PL 50, Fig. 9; PL 53, Fig.l 



A spherical, free-floating colony of 4-32 rhomboidal, reniform, or 

 broadly cuneate cells attached by a very short and broad stipe to 

 a common center; outer free wall straight, furnished at each angle 

 with 2 relatively short spines, 4-8^ long; cells 8-20/a in diameter, 

 6-18^ long, 5-8/i, thick.. 



Common in the plankton of many lakes; generally distributed. 

 Mich., Wis. 



FAMILY COELASTRACEAE 



This small family is characterized by cells that are radiately 

 arranged and adjoined, forming globular colonies which may be 

 either hollow or solid. In most species the cells are interconnected 

 by narrow processes or by extensions of the cell membrane to form 

 interstices. The chloroplast is parietal, covering nearly the entire 

 wall. Reproduction is by formation of a definite number of auto- 



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