170 PKDIASI l;l!MS OF THE UNITED STATES. 



tween the connecting end of one. and the base of the adjoin- 

 ing cells. Cells of the periphery deeply bilobed; lobes 

 conical or horn-like, sometimes acute, sometimes obtuse or 

 truncate. 



Frequent in pools everywhere. 



Var. BRACK YLOBUM, A. Br. Plate LXIV, fig. 35. 



Differs from the typical form in having the cells of the 

 periphery emarginate, notched, or shortly two . lobed, or 

 lobes almost obsolete; cells of the disc perforated with 

 smaller openings. 



Var. CLATHRATUM, A. Br. Discs pierced with larger openings ; 

 the lacunae being often as large as the cells. 



P. EHRENBERGII, (Corda.), A. Br. Plate LXIV, figs. 25-27. 



Marginal cells closely united, bilobed, medianly deeply 

 incised; each lobule with ends truncate and notched or in- 

 cised. Coenobium not perforated, composed of 4-8-16-32 

 cells. 



Var. CUSPIDATUM, A. Br. Plate LXIV, fig. 25. 



A small form, with lobes of cells finely, and often indis- 

 tinctly bidentate. 



Not so frequent as the preceding forms, hut widely dis- 

 tributed. 



P. TETRAS, Ehrb. Plate LXIV, fig. 24. 



Coenobium very small, four-celled, separated by colorless 

 interstices which form a cross. 



A. Braun. has a variety of P. Ehrenbergii, var. tniin-<iin,ii. 

 very near this form, but differs in having the ends of the 

 lobes notched our form, as far as observed, is perfectly 

 square, hence I retain the old name. 



Occurs frequently from Rhode Island to Florida. 



P. SELEN^EA, Kg. 



Coenobium orbicular; cells crescent-shaped, arranged in 

 one or more circles round one or two central cells. 

 A form reported by Bailey from Rhode Island. 

 P. CONSTRICTUM, HaSS. 



Coenobium nearly orbicular, continuous, composed of 

 16-32 cells ; cells of the periphery two lobed, or suddenly 

 contracted into two short cylindrical, obtuse processes. 



Bailey reports this species from South Carolina, Georgia 

 and Rhode Island. It is very nearly allied to Bori/amnn and 

 is probably a variety of that species. 



