184 FRESH- WATER ALG^E OF THE UNITED STATES. 



IsTot unlikely that some of the forms under this head also 

 belong to the arrested stages of development of higher forms. 

 Pringsheim observed resting spores in the development of 

 Hydrodictyon of polyhedron form, from which young Hydrodic- 

 tyon plants were developed. 



POLYEDRIUM TRIGONUM, 



Cells somewhat compressed, three to five-angled ; angles 

 obtuse mucronate, sides more or less concave. 



Kirchner suggests the following as names for varieties : 



a. typicum, Kirch. Three-angled, mucronate. 



b. minus, Eeinsch. Three or four-angled, sides concave, 



smaller than the last. 



c. tetragonum, Eab. Four-angled, ordinarily one mucro at 



each angle, rarely two or more. 



d.^pentagonum, Eab. Five- angled, otherwise as "c.' : 



e. -punctatum, Kirch. Four-angled ; each angle with a short 



obtuse process, membrane granularly roughened. 



f. bifurcatum, Wille. Four-angled, angles bifurcate sharply 



tipped. 



Diameter of cells, 13-36 /*. 



More or less frequent in pond waters. Of the six varieties 

 only such were identified as are illustrated. 



Plate CLIX, figs. 7-10, tetragonum; figs. 15-18, bifurcatumj 

 figs. 25-27, minus. 



POLYEDRIUM GIGAS, Wittr. 



Cells irregularly four or five -angled ; angles obtuse, sides 

 concave. 



Diameter of cells, 35-70 /*. 



Have reason to question whether this is not the form 

 recognized by Pringsheim as related to Hydrodictyon see 

 note above, with generic character. 



In ponds where Hydrodictyon prevails. 

 Plate CLIX, figs. 11-14, typical forms. 



POLYEDRIUM ENORME, (Ealfs) Eab. 



Cells irregularly four angled, with the angles produced, 

 hyaline, deeply bilobate; sometimes repeatedly bilobed, 

 with the ends of lobes acute, or mucronate. 



Diameter of cells, 25-40 ^. 

 Syn. Staurastrum enorme, Ealfs. 



