30 WISCONTSIN PHYTOPLANKTON 



var. ACHONDRUM (Boldt) W. & G. S. West. PI. 57, Figs. 5-6. 



Trans. Koy. Soc. Edinburgh 41: 484. 1905; Monogr. Brit. De^n. 2: 177, 

 pi. 62, figs. 6-9. 1905. 



Semicell shape subhexagonal, apices truncate and dorsal margins 

 with a distinct angularity instead of a smooth curve. (Tychoplank- 

 tont.) 



Cells 28 fi long ; 27 fx broad ; breadth of isthmus 7.5 fi ; thickness 

 11 /i. 



Found (rr), Sand (rrr), Sishebogema (rr). 



The alga as I have found it in Wisconsin lakes is somewhat smaller 

 than the cells of this variety that W. & G. S. West describe from 

 the Scottish plankton. The size and general shape of the Wisconsin 

 specimens is that of C. ahhreviatum var. planet onicum W. & G. S. 

 West but the broader semicells show that it should be referred to 

 C. depressum. 



3. CosMARiuM BiocuLATUM De Brebissou. PI. 57, Figs. 7-8. 

 in Ealfs, Brit. Desm. 95, pi. 15, figs. 5A-5F. 1848. 



CeUs very small, length and breadth about equal, deeply con- 

 stricted, sinus linear but open, with outer margin expanded and apex 

 rounded, isthmus narrow ; semicells transversely elliptic, with median 

 portion of dorsal and ventral margins flattened, lateral margins very 

 broadly rounded. Vertical view oblong-elliptic. Lateral view of 

 semicells circular. Cell wall smooth. Chloroplasts axial and with one 

 pyrenoid. (Tychoplanktont.) 



Zygospores spherical, armed with short blunt conical projections 

 whose length is about twice the breadth (Ralfs). 



Cells 18 [x long; 16-17 jn thick; breadth of isthmus 5.5 ju,; thickness 

 7.5 fi. 



Dummy (rrr). 



4. CosMARiuM coNTRACTUM Kirchner. PI. 57, Figs. 9-11. 



Kryptogamenfl. von Schlesiens 2^: 147. 1878; WoUe, Desm. U. S. 63, pi. 16, 

 fig. 1; pi. 50, fig. 24. 1884; W. & G. S. West, Monogr. Brit. Desm. 2: 170, pi. 61, 

 figs. 23-25, 34. 1905. 



Cells fairly small, length about one and a half times the breadth, 

 very deeply constricted, sinus open but with apex acuminate; semi- 

 cells elliptic-circular, with ventral margin more rounded than the 

 dorsal. Vertical view very broadly elliptic. Lateral view of semi- 

 cells circular. Cell wall smooth or very finely punctate. Chloroplasts 

 axial and with a single pyrenoid. (Facultative planktont.) 



Zygospores spherical to angular-spherical, smooth-walled (W. & 

 G. S. West). 



Cells 29-37 fi long ; 22-24 /x broad ; 5-6.5 ix broad at isthmus ; 

 15 /x thick. 



Diameter of zygospores 31.5-35 /u, (W. & G. S. West). 



Beaverdam (s), Carroll (r), Horseshoe (rr). Shell (s). 



