18 WISCONSIN PHYTOPLANKTON 



semicells with straight or undulate sides and with truncate apices 

 that are without ornamentation, inflated bases of semicells always 

 with short vertical plications that frequently have a granule at the 

 base of each plication. Cell wall smooth, finely punctate, or with 

 delicate vertical striae. Chloroplasts axial and with irregular longi- 

 tudinal ridges ; with 6-8 pyrenoids arranged in an axial series. 

 Zygospores unknown. 



In his British Desmidieae Ralfs includes both Pleurotaenium and 

 Docidium in the same genus. Lundell was the first to differentiate 

 clearly between the two and this characterization of the two genera 

 has been followed by W. & G. S. in their Monograph. Docidium 

 differs from Pleurotaenium in the vertical plications at the bases of 

 the semicells, the lack of ornamentation at the apices of the semicells, 

 and in the axial chloroplasts. 



DocmruM undulatum Bailey. PL 55, Fig. 4. 



Smithsonian Cont. to Knowl. 2, Art. 8: 36, pi. 1, fig. 2, 1851 (sep.) ; W. & G. S. 

 West, Monogr. Brit. Desm. 1: 194, pi. 27, figs. 7-10. 1904. 



Length about 15 times the breadth, cells very slightly constricted, 

 sinus widely open; semicells with sides strongly undulate (6-8 undu- 

 lations between isthmus and apex) and apex narrowly obversely 

 trapezoidal, angles of apex rounded and top flattened; basal plica- 

 tions ending in a small granule on the end towards the isthmus, about 

 six visible in front view of semicell. Cell wall smooth. Chloroplast 

 with an axial row of about eight pyrenoids. (Tychoplanktont.) 



Zygospores unknown. 



Cells 205 [x long ; 15 /x broad at basal inflation, 12.5 [i at apex. 



Diamond (rrr). 



TRIPLOCERAS Bailey 1851. 



Cells large, length several, times the breadth, median constriction 

 well defined but shallow; semicells cylindrical, always straight, some- 

 what attenuated from bases to poles, never compressed, with lateral 

 margins strongly undulate; undulations caused by transverse whorls 

 of mamillate protuberances that terminate in a single spine or 

 broadly emarginate verrucae; poles flattened and with two long, 

 upturned, divergent, obliquely disposed processes with truncate ends 

 that bear 2-3 short sharp spines (front view of polar processes giving 

 the cell apices a deeply incised appearance) ; apex with a protuberance 

 midway between bases of processes that bears one or two erect spines. 

 Cell wall, except for verrucae, smooth. Chloroplasts axial, with longi- 

 tudinal plates that extend from semicell base to apex; pyrenoids in 

 an axial row. 



Zygospores unknown. 



A highly differentiated genus that shows but little in common with 

 the other genera that have straight elongate cells. The apical 

 processes and transverse rows of verrucae on the cell wall are not 

 found in related genera. 



