WISC02^'SIN PHYTOPLANETON 125 



var. GLABRUM W. & G. S. West. PL 83, Fig. 15; PL 84, Figs. 3-5. 



Trans. Linn. Soc. 2ud. Ser. Bot. 5: 269, pi. 18, fig. 14. 1896. 



Processes frequently shorter and stouter, without the transverse 

 rings of granules, outline straight or somewhat undulate. Cell wall 

 smooth or punctate. (Facultative planktont.) 



Cells 105-120 IX long with processes, 62-65 ix long without proc- 

 esses; breadth with processes 106-115 /a, without processes 47-50 /a; 

 breadth at isthmus 25-26 [x. 



Black Oak (rrr), Des Moines (rrr), Devils (rrr). Fish (rr). Long (rrr), 

 Middle McKenzie (rrr). Mud Hen (rrr), Muskallonge (r). Poplar ,(sss), Reserve 

 (rrr), Rooney (rr), Sand (rr), Soft (ce). 



Intermediate stages between this variety and the type are some- 

 times found but generally the two are quite distinct. 



ARTHRODESMUS Ehrenberg 1838. 



Cells solitary, generally small, with the length usually equal to the 

 breadth, usually compressed and bilaterally symmetrical in front 

 view, rarely triangular in vertical view and with a radial symmetry, 

 median constriction pronounced and with sinus widely open to linear ; 

 semicells in front view generally triangular, rarely elliptic, sub- 

 trapeziform or subrectangular, with simple straight or curved spines 

 at the angles, spines lying in the same plane. Vertical view usually 

 elliptic and with spines at the poles, rarely triangular. Lateral view 

 of semicells subcircular. Cell wall of uniform thickness and without 

 ornamentation on body of semicells. Cells sometimes embedded in a 

 wide gelatinous sheath with blunt radial strands of denser mucilage. 

 Chloroplasts axial, with one pyrenoid. 



Zygospores spherical ; with walls smooth or bearing simple spines. 



This genus was orginally founded on the compressed nature of the 

 cells in combination with simple spines at the angles and lack of 

 ornamentation on the body wall. We know now, however, that there 

 are bilaterally symmetrical Staurastrum species with spines at the 

 angles of the semicells and smooth walls. W. & G. S. West have also 

 described radially symmetrical varieties of certain Arthrodesmus 

 species so that there is no sharp distinction between Arthrodesmus 

 and Staurastrum. The systematic treatment of the group is facili- 

 tated by considering it a separate genus and not a subgenus or sec- 

 tion of Staurastrum. On the other hand, species like A. octocornis 

 Ehrenberg and A. tenuissimus Archer approach some of the smaller 

 Xa7ithidium species. The central portion of the cell face in Xanthi- 

 dium is generally thickened or protruding, while in Arthdrodesmus 

 it is not differentiated. 



WtA\ 



