WISCONSIN PHYTOPLANKTON 133 



10. Arthrodesmus triangularis Lagerheim. PI. 86, Fig, 1. 

 of vers. Kgl. Sv. Vet.-Ak. Forh. 42, No. 7: 244, pi. 27, fig. 22. 1885; W. & G. S. 

 West, Ann. Eoy. Bot. Garden Calcutta 6: 211, pi. 15, fig. 7. 1907; Monogr. Brit. 

 Desm. 4: 97, pi. 114, figs. 11-13, 17. 1912. 



Cells small, length equal to or slightly greater than the breadth 

 (without spines), median constriction deep, sinus widely open and 

 with a blunt flattened apex, isthmus narrow and elongate; semicells 

 obversely triangular, with slightly convex sides and a distinctly con- 

 vex apex which is slightly retuse in the central portion; angles of 

 semicells somewhat pointed and each with a fairly long gradually 

 attenuated spine, spines parallel to somewhat but not markedly diver- 

 gent. Vertical view elliptic, with a single long spine at each pole. 

 Chloroplast axial and with a single pyrenoid. (Tychoplanktont.) 



Zygospores unknown in the type. 



Cells 15-23 ju, long ; breadth with spines 52-65 /x, without spines 

 14-22 /x; breadth at isthmus 5 fi; length of spines 18-22.5 /x. 



Dummy (rr), Tied Canoe (rrr). 



The type of this species is much scarcer than the varieties rotun- 

 datus and suhtriangularis. The species is characterized by the convex 

 apices of the semicells and the length of the spines. 



var. rotundatus (Raciborski) comb. nov. PI. 86, Figs. 2-4. 



Arthrodesmus incus var. rotundata Raciborski, Pamietnik Wydz. Akad. Umiej. 

 w Krakowie 17: 96, pi. 6, fig. 16. 1889; de Toni, Nuova Notarisia 5: 69. 1890; 

 Eichler & Eaciborski, Rozprawy Akad. Umiej. w Krakowie 2 Ser. 6: 120, pi. 3, 

 figs. 16-17. 1893. 



Arthrodesmus triangularis var. infiatus W. & G. S. West, Jour. Linn. Soc. Bot. 

 33: 320. 1898; Monogr. Brit. Desm. 4: 99 pi. 114, figs. 14-15. 1912. 



Arthrodesmus triangularis forma W. West, Jour. Linn. Soc. Bot. 29: 168, pi. 24. 

 fig. 19. 1892. 



Cells slightly larger than the type ; apices of semicells more convex 

 and indentation at center of apex very much reduced or lacking. 

 Elongation of isthmus not so conspicuous. (Facultative planktont.) 



Zygospores spherical, sparsely clothed with long attenuated spines. 



Cells 28-30 /^ long ; breadth with spines 70-76 ju, breadth without 

 spines 21 fi ; breadth at isthmus 6-7 /x ; length of spines 20-25 {x. 



Diameter of zygospores 24-30 ;u,; with spines, 13-15 ju, without 

 spines. 



Carroll (rr). Horseshoe (rr). Shell (rr), Streiter (rrr). 



"W. & G. S. West seem to have overlooked Arthrodesmus incus var. 

 rotundatus when they described their variety infiatus of A. triangu- 

 laris. The two are identical and Raciborski 's name for the variety 

 takes precedence. 



A few zygospores of this species were noted in one of the collec- 

 tions and this is the first record of zygospore structure in the species. 



