28 WISCONSIN PHYTOPLANKTON 



COSMARIUM Corda 1834. 



Cells never forming permanent colonies ; of variable size though 

 in the majority of cases fairly small, length never more than twice 

 the breadth, generally only slightly greater, in a few species with 

 breadth greater than the length; median constriction usually con- 

 spicuous and frequently linear ; cells compressed and with front view 

 of semicells quite variable in outline (circular, semicircular, elliptic, 

 ovate, trapezoidal, reniform or subquadrate), at times with the apex 

 elevated but never with the apical elevation incised, central area of 

 semicells at times protuberant. Vertical view generally elliptic, fre- 

 quently with an elevation in the median portion of the sides. Lateral 

 view of semicells most commonly circular. Cell wall smooth or 

 variously ornamented with granules of varying size or minute ver- 

 rucae, the ornamentation usually following a definite pattern; never 

 ornamented with spines. Chloroplasts usually single, axial, and with 

 four radiating plates and central pyrenoids; semicells at times with 

 two chloroplasts that have several radiating plates and each chloro- 

 plast with a single pyrenoid ; a few larger species with four parietal 

 chloroplasts each containing several pyrenoids. 



Zygospores generally spherical or ellipsoidal, rarely angular; 

 zygospore wall smooth or papillate to spinate. 



Certain of the species in this very large genus can scarcely be 

 distinguished from Euastritm ; other species resemble closely Penium 

 while the rare triradiate varieties might be considered Staurastra. 

 Euastrum is separated from Cosmarium by the apical incision. Cos- 

 marium differs from Penium in the compressed cells and from 

 Staurastrum in the lack of spines. 



Since the drawings in this work have been made from preparations 

 in which in many cases but a single cell of the species was available 

 the side and top views are lacking. These views are essential for 

 the critical interpretation of the various species and certain records 

 of the genus have been omitted since vertical and lateral views were 

 not available. In Wisconsin lakes Cosmarium is a comparatively 

 rare organism whereas in certain European lakes it may be present in 

 quantity. 



KEY TO THE SPECIES. 



Cell wall smooth or punctate, outline never granulate. 



Semicell outline truncate-pyramidate (5) C. granatum 



Semicell outline not truncate-pyramidate. 

 Sinus linear. 



Semicells semicircular (1) C. circulwre 



Semicells elliptic (2) C. depressvm 



Sinus open. 



Semicells subcircular (4) C. contractum 



Semicells broadly elliptic (3) C. bioculatum 



Cell wall ornamented and outline granular. 



Apices of semicells not elevated. . 



Cell apex flattened. 



