138 



FRESH-WATER BIOLOGY 



29 (30) Chromatophore a single axial plate with one or more pyrenoids. 



Mesotaenium Nageli. 



Cells cylindrical, with rounded ends, resembling in structure 

 A cells of Mougeolia but smaller, sometimes adhering to each other 

 after division but not forming distinct filaments. 



'^ — va'--^«*j ^~%v Fig. I2C. Mesotaenium endlkherianum KdigtW. X 625. a. showing 



;.^*^^t¥A~«! '^^fi^^'T^^^^J B the surface of the chlorophyll plate, b. showing the edge of the chloro- 

 phyll plate. (Original.) 



30 (29) In each cell-half several chlorophyll plates 31 



31 (32) Margins of radial plates entire; pyrenoids central in each cell-half. 



Penium de Bary. 



Cells sometimes slightly constricted at the middle, rounded or trun- 

 i^^^^'^^^X cated at the ends; length 3 to g times the breadth; membrane smooth, 



jl^ DgCrl punctate, or longitudinally striated; chromatophores radially placed 



- about a large pyrenoid in each cell-half. 



Fig. 121. Penium cucurbitinum Biss. X 295. (After West.) 



32 (31) Margins of the radial plates of the chromatophore scalloped ; pyrenoids 



several and scattered Netrium Nageli. 



Cells shaped much as in Penium. The scallops of the outer margin of the chromatophores 

 conspicuous; pyrenoids not large and forming a center about which the chlorophyll plates 

 radiate, as in Penium, but small and scattered. 



Fig. 122. Melrium lamelhsum Brebisson. X 200. (After Kirchner.) 



2,2, (23) Cells constricted at the center 34 



34 (42) Constriction at the sides slight and usually gradual 35 



35 (38) Length of cells usually not more than six times the breadth. . . 36 



36 (37) Central constriction very gradual and shallow; a slight incision at 



the ends Tetmemonis Ralfs. 



Cells straight, fusiform, or cylindrical, slightly and broadly 

 constricted at the middle; ends rounded, each with a slight 

 linear incision; length 4 to 6 times the diameter. Chroma- 

 tophore axial with a single row of pyrenoids. 



Fig. 123. Tetmemorus sranulatus Ra.Us. X 465- (Original.) 



37 (36) Cells short, ends truncate, constriction rather abrupt, but not deep; 



chromatophore of longitudinal bands; pyrenoids many, 

 scattered Pleurotaeniopsis Lundell. 



_ This is regarded by Brebisson as a Calocylindrus, by de Bary 



•^W^«!^l^^ «»^^^ ^^ ^ Pleurotaenium and by West as a Cosmarium. Formerly 



'^^j»^j£r^^3?^»%«^-^^S^ VVille recognized the genus, Pleuretaeniopsis, but now includes it 



• */ j4&_GL#>j?'- 33:, -^ under Cosmarium. 



Fig. 124. Pleurotaeniopsis lurgidus Lund. X 130- (After de Baxy.) 



