THE FRESH-WATER ALGAE 139 



38 (37) Length of cells many times the breadth 39 



39 (40, 41) Cells before the middle constriction swollen, but without longitu- 



dinal flu tings; chromatophore of radially-placed plates, with 

 pyrenoids Pleurotaenium Lundell. 



Cells straight, cylindrical, somewhat taper- 

 ing toward the truncate ends. Membrane 

 smooth or with small warts; at each end a 

 FIG. 125. Pleurotaenium nodulosum Rabenhorst. colorless vacuole with dancing particles as in 

 X 175. (Original.) Clostenum. 



40 (39, 41) Cells before middle constriction swollen and with longitudinal 



flutings; chromatophores of longitudinal radial plates. 



Docidium Lundell. 



Cells tapering somewhat towards the ends; no vacuoles with moving granules; membrane 

 either smooth or with minute protuberances and even with spines in certain regions. 



FIG. 126. Docidium baculum Brebisson. X 545. (Original.) 



41 (39, 40) Shape of cells much as in Pleurotaenium, but apices broadly cleft 

 or with bidentate processes ...... Triploceras Bailey. 



Cells large, walls covered with rings of furcate processes or small, perpendicular longitudi- 

 nally-placed plates. Sometimes_confused with Docidium. 



FIG. 127. Triploceras gracile Bailey. One-half of a cell. (After Cush man.) 



42 (34) Constrictions at the sides deep and abrupt .......... 43 



43 (44) End views of cells 3 to several angled or rayed. 



Staurastrum Lundell. 



Side view hour-glass shaped; membrane smooth or with 

 warts or spines; chromatophores in each cell-half consisting 

 of radially-placed plates about a central pyrenoid, two 

 plates extending into each arm or angle 



FIG. 128. Staurastrum crenulatum Nageli. X 600. (Original.) 



44 (43) End views of cells compressed or elliptical, often enlarged at the 



center .................. .... 45 



45 (48) Cells at end with notches or linear incisions .......... 46 



