WISCONSIN PHYTOPLANKTON 63 



number of subgenera. The simplest members of the genus resemble 

 triangular Cosmaria that are without ornamentation, in fact it is 

 impossible in certain eases to distinguish between varieties of Cos- 

 mariiim species that are triangular in vertical view and Staurastrum 

 species. These simple species lead up to those which have long 

 spines at the angles and then to those which have spines or denticu- 

 lations on both the angles and the cell body. The majority of the 

 species have the cell angles continued in hollow processes, the 

 triangular ones being the most numerous and probably the most 

 typical for the genus. On one hand some of the triangular species 

 have become compressed and form a group with a bilateral sym- 

 metry. On the other hand the radial symmetry of the species with 

 more than three processes is very striking. The climax of develop- 

 ment of processes is reached in those species in which there are two 

 distinct whorls of processes on each semicell. 



In addition to the close connection noted above between the 

 simple Staurastrum species and Cosmarium there is also a close 

 relationship between other Staurastrum species, particularly the 

 biradiate simple-spined species, and Arthrodesmus. The triradiate 

 varieties of one or two Xanthidium species also show a striking re- 

 semblance to Staurastrum species. 



The genus is by far the most important one of the family in the 

 plankton community. The total number of species and varieties 

 found compares favorably with the total number of species of all 

 the other genera of the Desmids, while the number of individuals 

 observed is greatly in excess of all the other Desmids. It is in this 

 genus that we find the largest number of forms which are true 

 plankton organisms while many more that are known from the 

 benthos seem to thrive even better under plankton conditions. In 

 general the species with long processes, probably because of the 

 greater buoyancy, seem best fitted for living in midlake, although 

 some of the species without processes but with a conspicuous gela- 

 tinous envelope are also well equipped for the plankton. 



KEY TO THE SPECIES. 



(A) Angles of cells not continued in hollow processes. 

 (B) Without spines at angles or on cell body. 

 Cell wall smooth. 



Isthmus not elongate. 

 Angles without mueros. 



Ventral margin less rounded than the dorsal. 



Cells small (3) S. muticum 



Cells large ( 1 ) S. grande 



Ventral margin more rounded than the dorsal (2) S. suhgrande 



Angles with mueros (4) S. irevispinum 



Isthmus elongate (5) S. pseudopachyrhynchum 



