PLATE XXI. 



FIGS. PAGE 



1 '2. Closterium prselongum Breb. 1, x 200 (after 



Brel>iss<m) ; 2, x 500 . . .164 



3-5. Cl. prselongum forma lircrior West. 3 and 4, 



x 400 ; 5, zygospore, x 200 . .165 



6-7. Cl. stri'yowm Breb. x 400 . . 165 



8-12. CL gracile Breb. x 520; 11 and 12, zygospores 166 

 13. Cl. gracile var. tenue (Leinm.) West & G. S. 



West, x 520 . . .168 



14-16. Cl. gracile var. elongatum West & Gr. S. West. 



x 400 . . . . 168 



17. CL Lnndellii Lagerh. x 400 . . 169 



18-19. CL aciculare Tuffen West, x 400 174 



