128 WISCONSIN PHYTOPLANKTON 



Cells 35-38 /A long with spines, 27.5-30 /x long without spines; 

 breadth with spines 68-85 [x, without spines 22-27 /x ; breadth at 

 isthmus 6-8 /x ; thickness 12 /x ; spines 22-26 /x long. 



Horseshoe (rrr), Meta (rr). 



This variety differs from the type in the great reduction in the 

 curvature of the portion of the semicell lying above the spines. In 

 this respect the shape of the semicells shows a close affinity with the 

 A. subulatus forma major figured by Nordstedt (Vidensk. Medd. f. d. 

 naturh. Foren. i Kiobenhavn 1869: 232, pi. 4, fig. 59. 1870) but the 

 Wisconsin cells being much smaller have been considered distinct 

 from the form major. 



4. Arthrodesmus michiganensis Johnson. PL 85, Fig. 4. 

 Bull. Torr. Bot. CI. 22: 296, pi. 232B, -fig. 12. 1895. 



Cells somewhat under medium size, length about one and a half 

 times the breadth (without spines), median constriction deep, sinus 

 open, with apex broadly rounded, isthmus narrow and elongate ; semi- 

 cells broadly reniform-pyramidate, with the basal halves obelliptic 

 and the apices truncate-pyramidate, flanks of truncate portion re- 

 tuse ; lateral angles of semicells broadly rounded and each with a long 

 somewhat attenuated curved spine, spines inwardly convergent or out- 

 wardly divergent. Vertical view elliptic, with a single straight spine 

 at each pole. (Tychoplanktont.) 



Zygospores unknown. 



Cells 37 jtx long ; breadth with spines 65 /x, without spines 27 /x ; 

 breadth at isthmus 5.5 /x ; spines 21-23 /x long. 



Soft (rrr). 



This rare alga has not been reported since it was discovered. The 

 specimens that I have seen are proportionally narrower than Johnson 

 figures them and the spines may be convergent. 



5. Arthrodesmus phimus Turner. PI. 85, Fig. 5. 



Kgl. Sv. Vet.-Ak. Handl. 25, No. 5: 136, pi. 12, iig. 9. 1893; W. & G. S. West, 

 Trans. Linn. Soc. 2nd. Ser. Bot. 6: 192, pi. 22, fig. 21. 1902; Monogr. Brit. Desm. 

 4: 104, pi. 115, -figs. 15-16. 1912. 



Cells small, breadth (without spines) somewhat greater than the 

 length, median constriction deep, sinus acute-angled and with the 

 apex acuminate, isthmus fairly narrow; bases of semicells obsemi- 

 circular, apices strongly retuse, lateral angles blunt and each ter- 

 minating in a single short pointed spine, spines divergent. Vertical 

 view narrowly elliptic, with a short spine at each pole. Chloroplasts 

 axial and with a single pyrenoid. (Tychoplanktont.) 



Zygospores unknown. 



Cells 36 /x long with spines, 25 /x long without spines ; breadth with 

 spines 42 /x, without spines 35 yu,; breadth at isthmus 7.5 /x; spines 10 /x 

 long. 



Clear Crooked (rrr), Owen (rrr), Sand (rrr). 



