40 WISCONSIN PHYTOPLANKTON 



If the alga is to be excluded from Cosmocladium I feel that Dic- 

 tyocystis should be retained as a separate genus, that, on account of 

 its axial chloroplast, is closely related to Asterococcus of the Palmel- 

 laceae. 



MICRASTERIAS Agardh 1827. 



Cells usually solitary, rarely in filaments, frequently of large size 

 and with the length somewhat greater than the breadth, median con- 

 striction always well defined and usually linear to sublinear, isthmus 

 never broad; bilaterally symmetrical and greatly compressed (in one 

 variety radially symmetrical) ; semicells semicircular to hexagonal, 

 with two incisions from periphery to about half way to center divid- 

 ing the semicell into three lobes— the central or polar lobe_ and two 

 lateral lobes; or with four incisions that divide the semicell into 

 one polar and four lateral lobes ; depth of incision variable and shape 

 linear to cuneate ; polar lobe generally with apex expanded and some- 

 what emarginate, at times with accessory asymmetrical processes; 

 lateral lobes rarely entire, usually with one, two, three or four series 

 of secondary lobelets, the ultimate lobelets generally with emarginate 

 apices or spines. Cell wall usually smooth, at times with rows of 

 spines near the incisions or marginal denticulations ; entire body of 

 cell rarely covered with spines. Chloroplast a flattened massive axial 

 plate having the same general contour as the semicell, with numerous 

 small ridges at right angles to the chloroplast body, lobe of chloroplast 

 in polar lobe of cell frequently deeply incised; pyrenoids numerous, 

 from thirty to a hundred, and fairly evenly distributed throughout 

 the chloroplast. Vertical view usually narrowly fusiform-elliptic and 

 lateral view of semicells narrowly pyramidate. 



Zygospores spherical, with long stout radially disposed spines that 

 terminate in simple to quadrifid apices. 



The genus comprises a heterogeneous range of cell shapes but the 

 incisions forming the lobes are a very constant feature in all species. 

 The cells are usually compressed and disc-shaped but the discovery of 

 a triradiate variety of M. pinnatifida by William West (Jour, of Bot. 

 27: 206, pi. 291, fig. 15. 1889) shows that even in this genus certain 

 qualifying statements must be made about the shape of the cell. 

 Another feature of Micrasterias species is the large cell size and, 

 although certain other genera have a few species that approach them 

 in size, large cells are the rule and not the exception in Micrasterias. 



KEY TO THE SPECIES, 



Cells not united in filaments. 



Polar lobe without accessory processes. 

 Semicells with two lateral lobes. 



Cells smaU (1) Af- pinnatifida 



Cells large. 



Polar incision acute-angled (2) M. laticeps 



Polar incision broadly rounded (3) M. depauperata 



