WISCONSIN PHYTOPLANKTON 181 



Differing from the foregoing in the triangular semieells, narrower 

 isthmus, as Avell as in the greater length and degree of convergence 

 of the spines. 



8. Arthrodesmus quiriferus "W. & G. S. West. PI. 85, Fig. 18, 



Jour. Linn. Soc. Bot. 35: 542, pi. 17, figs. 9-10. 1903; Monogr. Brit. Desm. 4: 

 101, pi. 115, figs. 6-7. 1912. 



Cells fairly large, length (without spines) slightly greater than 

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

 a rounded apex, isthmus narrow and slightly elongate ; semieells ob- 

 versety subtriangular, with the basal margins slightly convex and 

 apex markedly retuse ; angles of semieells somewhat rounded and 

 each furnished with a single extremely long gradually attenuated 

 spme, spines markedly divergent. Vertical view elliptic, with a long 

 spine at each pole. Chloroplasts single, axial, with one pyrenoid. 

 (Euplanktont.) 



Zygospores unknoAvn. 



Cells 50 fi long with spines, 35 /x long without spines ; breadth with 

 spines 91 fi, without spines 30 /x ; breadth at isthmus 7 /u, ; length of 

 spines 28 fi. 



Clear Crooked (rrr), Speese (rr), Squirrel (rr). 



The individuals that I have seen in Wisconsin lakes are not exactly 

 typical, but after a comparison of them with type material from 

 Loch Luiehart (Scotland) in the G. S. West collection I feel con- 

 vinced that they are a form of this species midway between the type 

 and the form compacta W. & G. S. West. Unlike the British speci- 

 mens the Wisconsin plants do not show a twisting at the isthmus. 



The species should be compared with Arthrodesmus incus (De Bre- 

 bisson) Hassall from which it differs in the longer spines and con- 

 cave apices of the semieells. 



9. Arthrodesmus incus (De Brebisson) Hassall. PI. 85, Figs. 19-22. 



Brit. Freshw. Algae 357, pi. 85, fig. 10. 1845; W. & G. S. West, Mouogr. Brit. 

 Desm. 4: 90, pi. 113, figs. 13-15. 1912. 



Cells small, length (without spines) about equal to the breadth, 

 median constriction deep, sinus widely open and with an acute apex, 

 isthmus narrow but not elongate ; semieells obversely triangular, with 

 straight or slightly convex sides, apex straight or somewhat concave, 

 angles of semieells acute and each with a single long gradually at- 

 tenuated spine, spines subparallel to strongly divergent. Vertical 

 view elliptic, with a long spine at each pole. Chloroplasts axial, with 

 a single pyrenoid. (Facultative planktont.) 



Zygospores structure not definitely known, probably spherical and 

 with simple spines (W. & G. S. West). 



Cells 27-50 fx. long with spines, 22-27 fx long without spines ; breadth 

 with spines 65-80 jx, without spines 21-26 /a ; breadth at isthmus 

 6-7.5 IX ; spines 25-30 fx long. 



Eed Bass (rrr), Chain (rr), Devils (r), Speese (r). 



