WISCONSIN PHYTOPLANKTON 57 



var. mediolaeve var. nov. PI. 65, Figs. 3-4. 



Xanthidium armatv/m var. cervicorne G. M. Smith non W. & G. S. West. Trans. 

 Wis. Acad. Sci., Arts & Lett. 20: 346. 1922. 



Cells larger than the type, length (without spines) about a quarter 

 greater than the breadth; concavity of lateral margins much more 

 pronounced ; processes at angle solid, much longer, with 3-5 deep 

 furcations, the ultimate furcations being quite sharp ; intramarginal 

 processes of superior lateral angles lacking; center of cell smooth, 

 without any sign of a process. (Tychoplanktont.) 



Cells 168-175 jx long with processes, 135-138 jx long without 

 processes; greatest breadth 120-123 )u, with processes, 92-98 /t without 

 processes ; isthmus 31-33 fx. broad ; processes 15-20 ju, long. 



Sunday (rrr). 



In the elegant antler-like processes of the angles this variety re- 

 sembles the variety cervicorne W. & G. S. West, to which Canadian 

 specimens were referred. Study of European specimens of the 

 varieties cervicorne and fissuni Nordstedt shows that in these two 

 varieties the central protuberance on the face of the semicell is readily 

 observed. Since this central protuberance is not found on the cells 

 that I have collected (both from Wisconsin and Ontario) the Ameri- 

 can alga is placed as a distinct variety. The intramarginal processes 

 of the superior lateral angles were also lacking in all specimens col- 

 lected but this feature is not of so great importance as the lack of 

 a central protuberance since the varieties fissum and cervicorne may 

 at times also lack the superior intramarginal processes. 



2. Xanthidium antilopaeum (De Brebisson) Kiitzing. PI. 65, 



Figs. 5-6. 



Species Algarum 177. 1849; W. & G. S. West, Monogr. Brit. Desm. 4: 63, pL 

 108, figs. 7-18. 1912. 



Cells of medium size, length (without spines) about equal to the 

 breadth, deeply constricted, sinus with inner portion linear and apex 

 very slightly dilated, outer portion widely open, isthmus fairly nar- 

 row; semicells with ventral margin semielliptic and dorsal margin 

 semihexagonal, lateral angles and angles of apex slightly rounded, 

 basal angles very broadly rounded, apex flattened and bearing two, 

 straight to slightly curved, suberect to widely divergent, fairly long, 

 acutely pointed spines at each lateral margin ; lateral angles each with 

 a pair of similar, horizontally disposed spines. Vertical view elliptic, 

 with broadly rounded poles that bear a pair of long divergent spines 

 and a second pair of similar intrapolar spines, lateral margins with 

 a slight median protuberance. Lateral view of semicells subspherical, 

 with a pair of divergent spines at the apex and a second pair in the 

 middle of the cell face. Cell wall finely punctate and with a median 

 thickened area that is variously scrobiculate and frequently brownish 

 in color. Chloroplasts generally four thin parietal plates in each 



