56 



WISCONSIN PHYTOPLANKTON 



KEY TO THE SPECIES. 



Spines broad furcate processes (1) X. armatwm 



Spines simple. 



Semicell with two spines on lateral margin (4) X. subhastiferum 



Semlcell with more than two spines on lateral margins. 



Each margin with two pairs of spines (2)X. antilopaeum 



Each margin with two pairs and a single spine (3) X, cristatum 



1: Xanthidium armatum (De Brebisson) Rabenhorst. 



Deutschl. Kryptogamenfl. 2, Abt. 2: 55. 1847 (Ref. Nordst. Index Desm.) ; 

 Ralfs, Brit. Desm. 112, pi. 18, figs. A-G. 1848. 



Cells large, length one and a quarter to one and a half times the 

 breadth, deeply constricted, sinus acute-angled, with apex acuminate 

 and outer margin widely open, isthmus narrow; semicells sub- 

 octagonal, with margins facing the isthmus slightly convex, lateral 

 margins concave, and apex plane; lateral and superior angles bear- 

 ing very stout, short, solid processes that terminate in 2-4 tuberculate 

 furcations, lateral margins with a single similar process within each 

 angle ; apex also with two similar subapical processes a short distance 



Fig. 6. Xanthidium armatum (De Brebisson) Rabenhorst (x600). From speci- 

 mens collected at Loch Cormisa, Skye, Scotland, by W. & G. S. "West. 



inward from the angle ; face of semicell with a single prominent solid 

 tuberculate process with a ring of marginal teeth. Vertical view 

 oblong-elliptic, with poles bearing four marginal and 3-4 intra- 

 marginal processes similar to those of the front view, middle of lat- 

 eral margins with a solid, broad, truncate process with several tuber- 

 cules at the distal end. Lateral view of semicells ovate-rectangular, 

 with a broad solid tuberculate process at the base, and with four 

 apical processes in addition to a median and basal transverse row of 

 processes. Cell wall punctate. Semicells with four parietal chloro- 

 plasts each containing several pyrenoids. 



Zygospores spherical, with thick walls containing numerous hem- 

 ispherical depressions (Ralfs). 



Cells 138 fi long with processes, 125 ju, long without processes; great- 

 est breadth with processes 91 ix, without processes 80 fi ; apex 57 ju, 

 broad with processes, 47 fj. without processes ; isthmus 34 fi broad ; 

 thickness 70 /x with processes, 58 fi without processes. 



Not found in the plankton of Wisconsin lakes. 



