130 



WISCONSIN PHYTOPLANKTON 



Fig. 17. Arthrodesmus Ealfsii W. "West (x700). From material in the Jenner 

 Herbarium of the British Museum of Natural History. 



Zygospores spherical, with simple long spines (Ralfs). 



Cells 16-18 /A long; breadth with spines 28-31 (U, without spines 

 about 16 fi ; breadth at isthmus 7.5-8.5 /x ; thickness 8-10 /a ; length of 

 spines 7.5-8.5 [j.. 



Dummy (rrr), George (rr), Horseshoe (rr), Pardee (rr). 



This alga was first considered a distinct species by William West 

 but later re-referred to Arthrodesmus incus where Ralfs originally 

 described it as an unnamed variety. 



I have examined type material collected by Ralfs and preserved in 

 the Jenner herbarium of the British Museum and feel that the alga 

 is sufficiently distinct to warrant the specific recognition that William 

 West first gave it. The most striking character is the convergence of 

 the spines. Arthrodesmus Balfsii also differs from A. incus (De 

 Brebisson) Hassall in the ratio between length and breadth as well 

 as in the shape of the semicells. 



The specimens from the Jenner herbarium are about one and a half 

 times the sizes of the Wisconsin specimens and are somewhat broader 

 than what W. & G. S. West consider typical A. Balfsii since they 

 also include the form latiuscula of W. & G. S. West. 



var. Brebissonii (Raciborski) comb. nov. PI. 85, Figs. 13-17. 



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

 w Krakowie 17: 96, pi. 6, fig. 15. 1889; de Toni, Nuova Notarisia 5: 68. 1890, 



Sinus widely open and with an acute, not rounded, apex ; isthmus 

 narrow; semicells obversely triangular, with the bases straight or 

 slightly tumid; spines much stouter and longer than the type and 

 more markedly convergent. (Facultative planktont.) 



Cells 21-27 /x long ; breadth with spines 47-60 fi, without spines 

 21-25/1; breadth at isthmus 6.5-7 /t; thickness 13 /a; spines 16-18 /x 

 long. 



Chain (rrr), Mud (sss). 



