WISCONSIN PHYTOPLANKTON 65 



With apical and subapical verrucae. 



Two subapical verrucae on each cell side (31) S. vestitum 



Four to seven subapical verrucae on each side (32) S. anatinum 



With apical verrucae only. 



Semicell base inflated and triangular (28) S. longiradiatum 



Semicell base not triangular (27) S. gracile 



(D) Vertical view not triangular. 

 (E) Semicells biradiate. 

 Semicells twisted at isthmus. 



Cells very small (33) S. tetracerum 



Cells of medium size (35) S. contortum 



Semicells not twisted at isthmus. 



Ends of processes all not in the same place. . (40) S. leptocladum var. sinautum 

 Ends of processes all in same plane. 

 Apex without verrucae or spines. 



Cell wall thicker in apical portion (37) S. americanum 



Cell wall of uniform thickness. 



Processes relatively short (33) S. tetracerum var. validum 



Processes long (36) S. chaetoceraa 



Apex with verrucae or spines. 



One spine or verruca on apex (40) S. leptocladum 



More than one spine or verruca on apex. 



Terminal spines of processes unequal (38) S. grallatoriv/m 



var. fordpigervm 

 Terminal spines of processes of same size. 

 Vertical view without a truncate projection. 

 Without ornamentation below apical verrucae. 



3 apical verrucae (38) S. grallatorium 



7-10 apical verrucae (39) S. anchora 



With ornamentation below apical verrucae (41) S. Johnsonii 



Vertical view with a truncate projection. 

 With a basal row of granules. 



Processes straight (42) S. natator 



Processes upwardly curved (40) S. leptocladum var. insigne 



Without a basal row of granules (43) S. urinator 



(E) Semicells with four or more processes. 

 Four processes on each semicell. 

 Outline of processes smooth. 

 Processes straight. 



Cell body gradually fusing with processes (22) S. brachiatum 



Cell body distinct at point of juncture (44) S. subnudibrachiatum 



Processes with a knee-like bend (45) S. inconspicuum 



Outline of processes not smooth. 



Vertical view without emarginate verrucae. 



Processes with concentric rings of granules (25) S. paradoxum 



Processes without rings of granules. . (47) S. arachne var. curvatum (p.p.) 

 Vertical view with emarginate verrucae. 

 Processes convergent. 



Vertical view with four verrucae on cell body (49) S. anlcyroides 



Vertical view with several verrucae (46) S. cerastes 



Processes subparallel to outwardly divergent. 



Upper margin only of processes crenulate (48) S. pentacerum . 



var. tetracerun 

 Both margins crenulate (47) S. arach-ni 



