WISCONSIN PHYTOPLANKTON 127 



resemblance to A. suhulatus Kiitzing but the latter has long parallel 

 to divergent spines and a different shape of the semicells. 



2. Arthrodesmus Bulnheimii Raciborski. PL 84, Figs. 7-8. 



Pamietuik Wydz. Akad. Uiniej. w Krakowie 17: 95, pi. 16, fig. 17. 1889; W. & 

 G, S. West, Mouogr. Brit. Desm. 4: 105, pi. 116, figs. 1-2. 1912. 



Cells of medium size, length (without spines) slightly greater than 

 the breadth, deeply constricted, sinus linear, isthmus narrow; semi- 

 cells transversely subrectangular, with lateral margins subparallel 

 and apex very slightly tumid, basal angles more rounded than the 

 apical angles, apical angles with long fairly stout gradually attenu- 

 ated spines, spines strongly divergent to suberect. Vertical view 

 elliptic, with a single straight fairly stout spine at each pole. Cell 

 wall smooth. Cells enclosed in a spherical gelatinous envelope with 

 a radially fibrillar structure. (Tychoplanktont.) 



Zygospores unknown. 



Cells 47-60 /x long with spines, 20-22 fi long without spines ; breadth 

 with spines 42-50 /x, without spines 20-23 /j. ; breadth at isthmus 5-8 ju, ; 

 spines 18-25 [x long. 



Lindy (rrr). 



The transversely subrectangular semicells combined with the con- 

 spicuous long spines, and especially the suberect position of these 

 spines, differentiate the alga from the others of the genus. 



3. Arthrodesmus subulatus Kiitzing. PI. 84, Figs. 9-10. 



Species algarum 176. 1849; W. & G. S. West, Monogr. Brit. Desm. 4: 109, pi. 

 116, fig. 14; pi. 117, fig. 1. 1912. 



Cells of medium size, length (without spines) slightly greater than 

 the breadth, median constriction deep, sinus widely open and with 

 the apex acute, isthmus narrow; semicells obsemicircular, with bases 

 convex and apices markedly tumid; lateral angles rounded and each 

 bearing a single long gradually attenuated spine, spines parallel to 

 slightly divergent. Vertical view elliptic, wdth a single spine at each 

 pole. Chloroplast axial, with a single pyrenoid. (Facultative plank- 

 tont.) 



Zygospores spherical, with smooth walls (W. & G. S. West). 



Cells 27-34 /x long; breadth with spines 53-80 yu., without spines 

 18-28 fi ; breadth at isthmus 6-7 /x ; spines 20-25 fx. long. 



Zygospores 30-36 ^li in diameter (W. & G. S. West). 



Diamond (rrr), Dummy (rrr), George (rr), Lindy (rrr). Tank (rrr), Travis 

 (r), Whitney (rrr). 



The distinguishing features of A. suhiUatus are the elevated apices 

 and the semicircular bases of the semicells. It shows a certain re- 

 semblance to A. convergens Ehrenberg but the two are quite distinct. 



var. Nordstedtii var. nov. PI. 85, Figs. 1-3. 



Apices of semicells slightly tumid, not strongly convex; spines al- 

 ways somewhat divergent. Vertical view narrowly elliptic. (Tycho- 

 planktont.) 



