142 WISCONSIN PHYTOPLANKTON 



radially fibrillar. Cells usually somewhat longer than broad, cylin- 

 drical to discoidal, median constriction very slight; ends of cells 

 plane and without projections, polar angles usually rounded. Verti- 

 cal view circular. Cell wall without ornamentation aside from trans- 

 verse ridges or in rare cases transverse rows of granules just beneath 

 the apices of the semicells. Chloroplasts axial and with several ver- 

 tical lobes that extend to the cell walls ; pyrenoids single and central. 

 Cells without a girdle-like thickening at the point of division. 

 Zygospores spherical and smooth-walled. 



This is distinguished from the other filamentous genera by the 

 cylindrical cells and lack of apical processes. Examination of ma- 

 ture cells might lead to the conclusion that there is no real distinction 

 between Hyalotheca and Desmidium but the manner of cell division 

 in the two genera is quite different. 



KEY TO THE SPECIES. 



Cells without a transverse subapical ridge (1) E. dissilieris 



Cells with a transverse subapical ridge (2) H. mucosa 



1. Hyalotheca dissiliens (Smith) De Brebisson. PI. 87, Figs. 8-9. 

 in Ralfs, Brit. Desm. 51, pi. 1, figs. lA-lI. 1848. 



Cells fairly small, breadth one and a quarter to two times the 

 length, very faintly constricted, sinus a slight annular depression 

 midway between the poles, breadth of isthmus essentially that of the 

 semicells ; cells cylindrical-discoidal, with flattened apices and slightly 

 rounded angles. Vertical view circular. Cell wall smooth, without 

 ornamentation aside from an annular ridge just above the isthmus. 

 Chloroplasts axial, with about six longitudinal rays that extend to 

 the cell walls; pyrenoids single and central; cells with but a single 

 chloroplast until nearly mature. Gelatinous colonial envelope broad. 

 (Facultative planktont.) 



Zygospores spherical, with smooth walls (Ralfs). 



Cells 10-20 fi long ; 20-27 /i broad. 



Bird (it), Carroll (rrr), Crab (rrr), Crooked (rrr), Devils (rrr), Howey (r), 

 Lynx (rr). 



2. Hyalotheca mucosa (DiUwyn) Ehrenberg. PI. 87, Fig. 10. 



Ber. Verh. Preuss. Akad. d. Wiss. Berlin 1840: 212. 1841; Ealfs, Brit. Desm. 53, 

 pi. 1, figs. 2A-2D. 1848; Lagerheim, Of vers. Kgl. Sv. Vet.-Ak. Forh. 1883, No. 

 2: 54. 1883. 



Cells fairly small, length slightly greater than the breadth, not 

 constricted ; shape of cells cylindrical, with the ends flattened and the 

 angles rounded. Vertical view circular. Lateral walls with a double 

 transverse subapical ridge at either end. Chloroplast axial, with 

 broad vertical rays; pyrenoid single and central. Gelatinous sheath 



