134 WISCONSIN PHYTOPLANKTON 



var. suBTRiANGULARis (Borge) W. & G. S. West. PI. 86, Figs. 5-7. 



Trans. & Proc. Bot. Soc. Edinburgh 23: 24, pi. 2, fig. 36. 1905. 



Arthrodcsmus incus var. suhtriangularis Borge, Bot. Not. 1897: 212, pi. 3, 

 fig. 4. 1897. 



Cells somewhat larger and with stouter spines; basal portion of 

 semicells indented just above the isthmus or with an irregular angu- 

 lar swelling, apex of semicells more elevated than the type and with a 

 pronounced indentation in the central portion, spines generally some- 

 what divergent. (Euplanktont.) 



Cells 35-43 fx long with spines, 30-32 /x long without spines ; breadth 

 with spines 75-78 /a, without spines 22-23 ^ ; breadth at isthmus 

 7-8.5 fi ; length of spines 25 ju,. 



Blue Gill (sss), Crab (rrr). South Crab (sss), Crooked (rrr). Little Doctor (ss). 



The variety differs in the irregularity of the outline of the basal 

 portion of the semicells. This irregularity is usually produced by 

 an indentation (Fig. 5) but individuals have been observed where 

 there is angular swelling instead of an indentation (Figs, 6-7) and 

 which, as far as the bases of the semicells are concerned, resemble 

 varieties of Arthrodesmus Ralfsii W. West. 



11. Arthrodesmus octocorxis Ehrenberg. PI. 86, Figs. 8-12. 



Die Infusionsthierchen 152. 1838; Wolle, Desm. U. S. 97, pi. 24, figs. 21-23. 

 1884; W. & G. S. West, Monogr. Brit. Desm. 4: 111, pi. 117, figs. 6-10. 1912. 



Xanthidium (?) octocorne (Ehrenberg) Ralfs, Brit. Desm. 116, pi. 20, figs. 

 2A-2E. 1848. 



Cells small, length (without spines) about one and a quarter times 

 the breadth, median constriction fairly deep, sinus semicircular, 

 isthmus relatively broad; semicells trapeziform, with concave bases 

 and lateral margins markedly incurved, the four rounded angles of 

 semicells bearing a single long gradually attenuated spine, spines on 

 lower angles parallel, those on upper angles divergent. Vertical view 

 elliptic, with two spines inserted one above the other at each pole. 

 Cell wall of uniform thickness and without ornamentation. Chloro- 

 plasts axial, with one pyrenoid. (Facultative planktont.) 



Zygospores spherical to subspherical, with a few simple somewhat 

 attenuated spines (W. & G. S. West). 



Cells 35-44 /x long with spines, 14-20 [x long without spines ; breadth 

 with spines 25-38 n, without spines 10-16 jj. ; breadth at isthmus 5-6 [i ; 

 thickness 6.5 fi ; length of horizontal spines 8-19 ij., length of oblique 

 spines 11-17 ju.. 



Zygospores 28 jx in diameter with spines, 15 fx, without spines (W. 

 & G. S. West). 



Blue Gill (rrr), Speese (rrr), Travis (c). 



At first glance this species appears to be a minute Xanthidium 

 but phycologists have excluded it from Xanthidium because there is 

 no thickening of the cell wall on the face of the semicells. It is a 



