74 WISCONSIN PHYTOPLANETON 



broadly rounded, isthmus narrowly elongate and subrectangular ; 

 semicells obversely subsemicircular, with dorsal margin slightly tumid, 

 angles acute to rectangular and furnished with a single short acute 

 spine, spines strongly outwardly divergent (suberect). Vertical view 

 triangular, sides of cell retuse in median portion, angles acute, fur- 

 nished with a single short straight acute spine. Cell wall smooth, 

 Chloroplast with a small central mass and two conspicuous laminate 

 blades leading to each angle of the cell; pyrenoid single and central. 

 (Facultative planktont.) 



Zygospores spherical, furnished with numerous stout straight 

 spines with broad bases and rounded apices (Ralfs). 



Cells 25-32 fi long with spines, 25-28 ix long without spines ; breadth 

 29-41 fx with spines, 24-32 /x, without spines ; isthmus 5.5-6.5 jj. broad ; 

 spines 2.5-5 [x long. 



Zygospores 36 /a in diameter, spines 12-20/1, long (Ralfs). 



Carson (r), Lac du Flambeau (rrr), Big McKenzie (rrr), Middle McKeuzie 

 (rrr), Mud (rr), Travis (sss). 



The distinctive features of this species are the elongate isthmus 

 and the upwardly slanting, comparatively small spines. In the 

 elongate isthmus it resembles 8. cuspidatum De Brebisson but the 

 length of the spines and broader semicells readily distinguish the 

 two. The cell shape is somewhat variable and at times the peculiar 

 isthmus does not stand out as prominently as in Ralfs' figures. 



var. iNFLATUM W. West. PL 68, Figs. 25-26. 



Jour. Linn. Soc. Bot. 29: 170, pi. 22, fig. 11. 1892; W. & G. S. West, Trans. 

 Eoy. Irish Acad. 32: 44, pi. 2, fig. 23. 1902. 



Cells somewhat larger, sinus with a fairly acute apes and isthmus 

 not elongate; apex of semicells more tumid. Vertical view with very 

 short spines at angles and with sides more retuse. (Tychoplanktont.) 



Cells 31-34 ju, long; breadth with spines 36-41 /x, without spines 

 31-35 ja; isthmus 6-7 fi broad; spines 2-4 ^u, long. 



Fish (rrr). 



Differing from the type in the lack of elongation at the isthmus 

 and the rounded dorsal margins of the semicells. The short outwardly 

 divergent spines and ventral sides of the semicells are like those of 

 S. dejectum so that the variety is well placed although it does not 

 have the characteristic isthmus. 



13. Staurastrum cuspidatum De Brebisson. PI. 68, Figs. 27-34. 



in Meneghini, Linnaea 14: 226. 1840; Ralfs, Brit. Desm. 122, pi. 21, figs. lA-lE; 

 pi. 33, fig. 10. 1848. 



Cells small, breadth (with spines) about twice the length, deeply 

 constricted, sinus widely open and with apex broadly rectangular, 

 isthmus narrow, rectangular, greatly elongated; semicells obversely 



