Crucigenia truncata G. M. Smith 1920, p. 146 

 PI. 66, Fig.2 



Colony free-floating, consisting of 4 ovate cells arranged about a 

 central, rectangular space, with lateral walls of each pair of cells 

 adjoining and with apical walls in contact at the corners, the outer 

 free walls thick and truncate, straight; chloroplasts 1-4, parietal 

 discs; pyrenoids absent; quartets of cells adjoined to form compound 

 colonies by the adherence of old mother cell walls; cells 4-6/x in 

 diameter; colonies up to 50/x wide, 75/x long. 



Rare in the plankton of several lakes. Wis. 



TETRASTRUM Chodat 1895a, p. 114 



Plant a free-floating colony of 4 triangular, cruciately arranged 

 cells, inner faces straight and adjoined to form a rectangular plate, 

 outer free wall convex and beset with 3 or 4 short setae; colony 

 inclosed by a thin gelatinous matrix; chloroplasts 1-4 in each cell, 

 parietal discs with pyrenoids sometimes absent. 



This plant should be compared with Crucigenia, a genus with 

 which it has much in common. Some species of Crucigenia in the 

 past have been assigned to Tetrastrum. The latter genus is character- 

 ized by the setae, by its possession of a definite colonial investment, 

 and by its failure to form compound colonies. 



Key to the Species 



Outer faces of cells with short, knob-Hke projections T. punctatum 



Outer faces of cells with long, hair-like setae T. staurogeniaejorme 



Tetrastrum punctatum (Schmidle) Ahlstrom & Tiffany 1934, p. 504 



Colony of 4 angular, or somewTiat triangular cells, broadly convex 

 on their outer free walls, arranged about a very small central space; 

 walls furnished with several coarse knob-like projections; chloro- 

 plast parietal and plate-like, with a single pyrenoid; cells 8-1 2/x in 

 diameter. 



Uncommon; in plankton. Mich., Wis. 



Tetrastrum staurogeniaejorme ( Schroeder ) 



Lemmermann 1900d, p. 95 



PI. 66, Fig. 3 



A colony of 4 triangular cells, cruciately arranged about a small 



rectangular space; lateral margins of the cells straight and adjoined, 



the outer free walls convex and furnished with as many as 6 fine, 



hair-like setae; chloroplasts 1-4 parietal discs, sometimes containing 



[286] 



