WISCONSIN PHYTOPLANKTON 145 



Ralfs thought that the large foramen between the cells of the fila- 

 ments in this species and Z>. Bailey i (Ralfs) Nordstedt justified plac- 

 ing them in a separate genus (Aptogoniim) but Aptogonum is not 

 recognized by phycologists of the present generation. 



3. Desmidium Baileyi (Ralfs) Nordstedt. PI. 88, Figs. 5-7. 



Lunds Univ. Arsskr. 16: 4. 1880 (sep.); Kgl. Sv. Vet.-Ak. Handl. 22, No. 8: 25, 

 pi. 2, figs. 4-5. 1888. 



Aptogonum Baileyi Ealfs, Brit. Desm. 208, pi. 35, figs. lA-lC. 1848. 



Cells united to form filaments that are straight (not spirally 

 twisted) and devoid of a gelatinous sheath. Cells fairly small, with 

 the length and breadth about equal, median constriction reduced to 

 a faint undulation ; cell outline rectangular, with the lateral margins 

 parallel and apices with a broad deep semielliptic depression that 

 gives the appearance of short erect truncate projections at either 

 lateral margin of the semicells apices. Adjacent cells of filaments 

 almost completely separated by elliptical to subrectangular openings. 

 Vertical view triangular, with slightly convex sides and rounded 

 angles. Cell wall smooth. (Facultative planktont.) 



Zygospores spherical and with smooth walls (Ralfs). 



Cells 20-26 fi long ; 21-27.5 ja broad. 



Beaverdam (rrr), Crooked (rrr). Diamond (rrr), Found (r), Little Doctor 

 (rrr), Muskallonge (rrr), Speese (rrr). 



Sharply differentiated from the other species by the rectangular 

 cells in which there is only the faintest indication of a median con- 

 striction. The filaments of this species, like those of D. aptogonum 

 De Brebisson, have elliptic open spaces separating adjacent cells. 



4. Desmidium Grevillii (Kiitzing) De Bary. PI. 88, Fig. 8. 



Unters. u. d. Fam. d. Conj. 42, pi. 4, figs. 30-31. 1858. 



Desmidium cylindricum Greville, Scott. Crypt. Fl. 5: pi. 293. 1827. 



Didymoprium Grevillii Kiitzing, Phycol. gen. 166. 1843; Ealfs, Brit. Desm. 57, 

 pi. 2, figs. A-K. 1848. 



Cells united to form filaments that are spirally twisted (about one 

 complete turn in every ten cells) ; filaments enclosed in a broad firm 

 gelatinous envelope; envelope frequently with radial fibrillae. Cells 

 of medium size, breadth about twice the length, slightly constricted, 

 sinus a small semicircular depression, isthmus very broad; semicells 

 trapezoidal, basal angles very broadly rounded, apical angles obtuse ; 

 lateral margins frequently with a sharp projection midway between 

 base and apex, the projections on opposite sides being connected by 

 a faint line; apex flat, without open spaces between adjacent cells. 

 Lateral view of cells octagonal, breadth about one and a half times 

 the length, without a constriction in the median portion. Vertical 



