38 WISCONSIN PHYTOPLANKTON 



The cells of this genus have the same shape as those of Cosmarium 

 but the organization into a definite colony differentiates Cosmocladium 

 from Cosmarium. With one exception this is the only colonial genus 

 of the family in which cell division takes place in three planes and 

 so gives rise to a colony that is not a filament. The gelatinous en- 

 velope enclosing the colony can only be demonstrated by special 

 methods but the gelatinous strands connecting the individual cells 

 are readily discernible. These strands have been the subject of spe- 

 cial investigations by Schroder and Liitkemiiller and they have shown 

 that the two sister cells are connected by strands that arise near the 

 isthmus of the mother cell at the time of division. These investigators 

 have also demonstrated the presence of definitely arranged pores in 

 the cell walls and Schroder has figured radially arranged denser 

 strands in the colonial envelope, these denser strands apparently orig- 

 inating at the pores of the cell wall. 



KEY TO THE SPECIES. 



Cells strongly compressed (1) C. saxonicum 



Cells not compressed (2) C Hitchcockii 



1. Cosmocladium saxonicum De Bary. PI. 58, Fig. 2. 

 Flora 48: 329, pi. 4, figs. 1-3. 1865; Schroder, Ber. d. D. bot. Ges. 18: 15, pi. 1, 

 figs. 1-9. 1900. 



Cells fairly small, length somewhat greater than the breadth, me- 

 dian constriction relatively deep, sinus sublinear, with apex rounded 

 and outer margin widely open, isthmus fairly narrow ; semicells trans- 

 versely elliptic to subhemispherical, with dorsal margin more rounded 

 than the ventral margin, lateral angles broadly rounded. Vertical 

 view oblong-elliptic, with sides subparallel and poles somewhat flat- 

 tened. Lateral view of semicells subcircular. Cell wall smooth. 

 Chloroplast axial, with a single central pyrenoid. Colonial envelope 

 a hyaline gelatinous matrix in which there are fairly broad denser 

 strands connecting the cells. (Facultative planktont.) 



Zygospores unknown. 



Cells 22-30 /x long; 21-26 /x broad; 7-7.5 /x broad at isthmus; 

 12-14 [I thick. Colonies up to 275 /x in diameter. 



Beaverdam (rr), Devils (ccc). Squirrel (rr), North Turtle (sss). 



The cells of the species as I have found them in Wisconsin have 

 more rounded semicells than De Bary or Schroder figure. The con- 

 necting threads also appear to be the same diameter throughout and 

 do not have the expansion midway between the two cells that these 

 investigators have noted, 



2. Cosmocladium Hitchcockii (Wolle) comb. nov. PI. 58, Figs. 3-9. 



Dictyosphaerium Hitchcockii Wolle, Bull. Torr. Bot. CI. 12: 126. 1885; 

 Preshw. Algae of U. S. 186, pi. 157, fig. 12. 1887. 



