WISCONSIN PHYTOPLANKTON 55 



Cells of medium size, united in filaments by the interlocking of 

 the polar lobe, rectangular in outline, very deeply constricted, sinus 

 sublinear, isthmus very narrow; semicells five-lobed; polar lobe ex- 

 serted, basal fourth of the polar lobe narrow, with erect parallel sides, 

 upper three quarters of the lobe greatly expanded, anvil-shaped, with 

 a very broad deep subrectangular excavation in the median portion 

 of the apex, lateral angles of the polar lobe acuminate, lateral mar- 

 gins of the base of the apical excavation with two erect asymmetri- 

 cally disposed spines of unequal length; lateral lobes asymmetrical, 

 the superior lobes divergent, the inferior lobes horizontally disposed ; 

 superior lobe incised, with the lobelet bordering the polar incision 

 simple and acuminate, with the other lobelet having an incision about 

 half the depth of the primary lobular incision ; all ultimate lobelets, 

 except those bordering the polar incisions with truncate-emarginate 

 apices. Cell wall smooth. Vertical view narrowly fusiform, with 

 apices acuminate and median portion somewhat constricted. Chloro- 

 plast completely filling the cell, with the portion in the polar lobe en- 

 tire, without conspicuous ridges; pyrenoids 10-15. (Tychoplank- 

 tont.) 



Zygospores spherical, furnished with rather numerous, fairly long 

 spines with truncate-emarginate apices (Wallich). 



Cells 78-90 fx. long; 82-85 /x broad, 45-58 /x broad at the apex of 

 the polar lobe ; 15-20 /x broad at the isthmus. 



Zygospores 45 yu in diameter (Wallich). 



Des Moines (rrr). 



XANTHIDIUM Ehrenberg 1837. 



Cells of variable, though generally of medium size; length fre- 

 quently somewhat greater than the breadth, median constriction deep 

 and sinus linear to acute-angled; semicells almost universally com- 

 pressed, very rarely radially symmetrical, of variable shape, though 

 usually polygonal and with apex flattened but not incised; lateral 

 margins with angles that bear simple, rarely furcate, spines; spines 

 asymmetrically disposed with reference to the vertical plane. Ver- 

 tical view elliptic. Lateral view of semicells circular to polygonal. 

 Cell wall smooth to punctate, with a thickened protuberant area in 

 the middle of the cell face that frequently is scrobiculate. Chloro- 

 plasts at times axial and two in each semicell, at other times parietal 

 and four in each semicell; pyrenoids usually single in each chloro- 

 plast. 



_ Zygospores spherical, with scrobiculate walls or with walls bearing 

 simple to furcate spines. 



The angular semicells with spines at the angles and the differ- 

 entiated central area of the cell are the most striking characters of 

 the genus. Except for certain rare triradiate varieties the cells are 

 always compressed. These varieties show a close affinity with 

 Staurastrum, while the forms with reduced spines may resemble 

 Arthrodesmus. 



