160 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



2. Chromatophore entire or halved. 3. 



3. Ends rounded. i. O. Borgei. 



3. Ends acute or subacute. 2. O. lacustris. 



1. O. BORGEI Snow, 1903, PI. II, fig. 7. Cells ellipsoidal or 

 slightly fusiform, ends rounded, wall uniform or slightly thick- 

 ened at one end, with no projection, 9-13 /x long ; chromatophore 

 single, with pyrenoid ; cells in colonies of two or a multiple of 

 two, free or united in a common gelatine. In plankton, Lake 

 Erie. Sweden. 



2. O. LACUSTRIS Chodat, 1897, p. 296; 1902, p. 190, fig. 

 103. Cells broadly fusiform, with membrane somewhat thick- 

 ened at the subacute ends, chromatophore single or in distinct 

 halves, without pyrenoid ; mother cell wall persistent, retaining 

 the characteristic form. In plankton, Lake Erie. Europe. 



3. O. SOLITARIA Wittrock in Wittr. and Nordst., Alg. 

 Exsicc., No. 244; P. B.-A., No. 1178. Cells generally soli- 

 tary, occasionally 2-4 together, ellipsoid, 13-35X7-18 /x, with 

 several chromatophores ; membrane thickish with a tubercular 

 projection at each end. Fig. 44. Greenland, Me., Mass., 

 Conn., Cal., West Indies. Europe, So. America. 



Forma MAJOR Wille, 1879, p. 26. Cells about 40X22/1*; 

 often in 4-celled colonies, 60X50 p.. Alaska. Northern Europe. 



4. O. CRASSA Wittrock in Wittr. and Nordst., Alg. Exsicc., 

 No. 355. Cells usually in twos, ellipsoid to fusiform, I4-23X 

 10-18 /A, with several chromatophores ; membrane thin, only 

 slightly thickened at the ends. Me., Mass. Europe. 



5. CHODATELLA Lemmertnann, 1898, p. 309. 



Cells ovoid or ellipsoid, the membrane bearing two to many 

 setae not thickened at the base ; chromatophores one or many, 

 parietal, with or without pyrenoid ; cells free or contained in 

 the mother cell wall. Reproduction as in Oocystis. 



This genus differs from Oocystis only by the presence of the 

 setae. Several European species have been described and may 

 be expected here ; the following species, the only one yet 

 reported here, is known only from Lake Erie. 



C. CITRIFORMIS Snow, 1903, p. 381, PI. II, fig. VIII. Cells 

 ellipsoidal with an obtuse projection at either end, i3-23X8-2O/x; 

 chromatophore single, with pyrenoid ; setae slender, in whorls 

 about the projections at the ends of the cells. Fig. 45. Lake 

 Erie. 



6. NEPHROCYTIUM Nageli, 1848, p. 79. 

 Cells somewhat curved, ovoid or reniform ; asexual reproduc- 



