Tetradesmus wisconsinense G. M Smith 1913, p. 76 

 PI. 64, Figs. 12-14 



Cells fusiform arranged with their long axes parallel; cells ad- 

 joined throughout the length of their lateral walls; outer free walls 

 concave; poles of the cell narrowed and directed away from the 

 center of the colony; in end view spherical, the cells arranged in 

 a quadrangle; cells 4-6/i, in diameter, 12-14. S/x long. 



Rare; in the plankton of lakes. Wis. 



CRUCIGENIA Morren 1830, pp. 404, 415 



Colony free-floating, consisting of a plate of 4-8-16 tiapezoid or 

 rhomboid cells lying in one plane about a small or large central 

 space; chloroplast a parietal plate with 1 pyrenoid in each cell; 

 colony inclosed by a thin, inconspicuous gelatinous envelope which 

 often causes the families to adhere to one another and to form 

 colonial complexes. 



Key to the Species 

 1. Opening at the center of the colony large, approximately 



equal to the greatest dimension of the cells 2 



1. Opening at center of the colony small, less than 



the greatest dimension of the cells 3 



2. Cells nearly hemispherical, old mother cell 



walls persistent — _ C. Lauterbornii 



2. Cells trapezoidal; outer free walls slightly convex; old 



mother cell walls not persistent .C. fenestrata 



3. Cells triangular; opening at the center of the colony very small 4 



3. Cells ovate or polygonal 5 



4. Outer free walls straight C. tetrapedia 



4. Outer free walls convex C. quadrate 



5. Cells with an apiculation at the free pole C. apiculata 



5. Cells without an apiculation 6 



6. Cells abruptly truncated at the free poles, the 



walls thickened at the apex C. truncata 



6. Cells not truncated, the wall not thickened at the apex 7 



7. Cells somewhat reniform, the outer free walls concave; 



cells always quadrately arranged C crucifera 



7. Cells ovate 8 



8. Cells regularly arranged in quartets or in multiples 



of quartets, 5-10/a long C. rectangularis 



8. Cells irregularly arranged within the colony, 



some in 4's, 8-14/j. long - C. irregularis 



Crucigenia apiculata (Lemm.) Schmidle 1901, p. 234 



PL 65, Fig. 3 

 Colony composed of 4 ovate, rhomboidal or somewhat tiiangular 

 cells arranged about a 4-sided opening, with 1 short, cone-shaped 

 apiculation on the cell wall at the free outer apex, and one on the 



[ 283 ] 



