WISCONSIN PHYTOPLANKTON 59 



The only difference between this variety and the foregoing is the 

 small erect spine associated with the verrucae below the apices of the 

 semicells. 



var. DEPAUPERATUM W. & G. S. West. PI. 66, Fig. 1. 

 Trans. Bot. Soc. Edinburgh 23: 23, pi. 1, figs. 15-16. 1905. 



Semicells with angles more rounded than in the type; spines on 

 lateral margins much smaller and not regularly arranged in pairs, 

 number of spines on a semicell frequently reduced. Central area 

 with scrobiculations in an irregular ring. (Facultative planktont.) 



Cells 68 fx long with spines, 65 /x long without spines ; 84 /x broad 

 with spines, 66 fi broad without spines ; isthmus 27.5 /x broad ; spines 

 7.5-10 IX long. 



Otter (rr). 



3. Xanthidium cristatum De Brebisson. PI. 66, Figs. 2-3. 



in Ealfs, Brit. Desm. 115, pi. 19, figs. 3A-3F. 1848; Eeinsch, Cont. ad Alg. et 

 Fung. 92, pi. 16, fig. 13. 1875. 



Cells of medium size, length (without spines) somewhat greater 

 than the breadth, deeply constricted, sinus sublinear to slightly open, 

 with apex somewhat inflated, isthmus narrow; semicell elliptic- 

 hexagonal, with sides facing the isthmus somewhat convex, lateral 

 margins flattened, and apex plane; apical angles each with a pair of 

 slightly divergent, medium sized, acutely pointed spines; superior 

 lateral angles with a similar pair of somewhat divergent spines; in- 

 ferior lateral angles with a single spine, basal spines of semicells con- 

 vergent; face of semicell with a small, circular to elliptic, thickened 

 area that is without ornamentation. Vertical view elliptic, with poles 

 subtruncate and bearing three marginal and two intramarginal 

 slightly divergent spines; lateral margins with a slight protuberance 

 in the median portion. Lateral view of semicells subspherical, with 

 a protuberance on each side and divergent apical and subapical spines. 

 Cell wall finely punctate. Semicells with four parietal laminate 

 chloroplasts, each containing a single pyrenoid. (Tychoplanktont.) 



Zygospores spherical, with long spines that have parallel sides and 

 truncate apices (Reinsch). 



Cells 65 ju, long with spines, 45-48 fi long without spines ; 48 fi 

 broad with spines, 35 fi broad without spines ; breadth of isthmus 

 10-12.5 JU, ; length of spines 5-10 /*. 



Zygospores 50.4 jn in diameter; spines 16.8-19.6 ju, long (Reinsch). 



Chetek (rrr), Found (rrr). Island (rrr), Lac Court Oreilles (rrr). 



The single convergent spines at the basal angles of the semicells 

 differentiate this species from all others. The central protuberance 

 is easily overlooked since it is without ornamentation. 



