THE GREEN ALGAE OF NORTH AMERICA 179 



7. P. DUPLEX Meyen, 1829, p. 772, PI. XLJII, figs. 6-20; 

 P. pertusum Wolle, 1892, PL L,XIV, figs. 33, 34; Wittr. and 

 Nordst., Alg. Exsicc., No. 1237. Cells 8-32, 6-28 p. wide, form- 

 ing a disk perforate with many rounded openings of varying 

 size ; disk cells quadrangular, or emarginate on one or more 

 sides ; marginal cells joined only at the base, deeply bilobed, 

 lobes straight, each ending in an. acute or obtuse, not capitellate 

 prolongation. Mass., Me., N. J., 111., Neb. Europe. 



A common species, and with many varieties ; but fairly well 

 recognized by the numerous rounded openings. 



Var. CLATHRATUM A. Braun, 1855, p. 93 ; Wittr. and Nordst., 

 Alg. Exsicc., No. 1562. Disk cells deeply emarginate, making 

 the openings proportionally larger than in the type. Mass., 

 Neb. Europe. 



Var. BRACHYLOBUM A. Braun, 1855, p. 93, PI. VI, fig. 25; 

 Wolle, 1892, PI. L-XIV, fig. 35. Disk and marginal cells only 

 slightly emarginate ; openings and marginal projections incon- 

 spicuous. Me. Europe. 



8. P. TETRAS (Ehrenb.) Ralfs, 1844, p. 469, PL XII, fig. 4; 

 P. Ehrenbergii A. Braun, 1855, p. 97, PL V.H. ; Wolle, 1892, 

 p. 170, PL LXIV, figs. 25-27. Cells 4-16, 9-20 /x wide, disk 

 cells polygonal, one side repand or with a very deep and narrow 

 incision ; marginal cells very deeply incised, lobes emarginate, 

 bidentate or bifid. Me., Mass., N. J., Ohio, 111., Neb., Colo., 

 Porto Rico, Mexico. Europe, Asia, So. America. 



The small continuous disk, with deep linear incisions in 

 nearly every cell, and with cell lobes divided, easily identifies 

 this species. 



9. P. BIRADIATUM Meyen, 1829, p. 773, PL XLJII, figs. 

 21-22; P. rotula A. Braun, 1855, p. 101, PL VI, figs. 1-3. 

 Cells 8-32, 9-21 p wide ; disk cells deeply incised, leaving rather 

 large openings ; marginal cells attached at the base only, in- 

 cised to or below the middle, each division ending in two acute 

 or obtuse denticulations. Greenland. Europe. 



From P. duplex, which has a similarly perforate frond, this 

 can be distinguished by the double division of the marginal 

 cells, giving two projections to each lobe. 



Var. EMARGINATUM (A. Braun) L,agerheim, 1882, p. 54; 

 P. rotula var. emarginatum A. Braun, 1855, p- 102, PL VI, figs. 

 4, 8, 9, ii. Disk cells slightly sinuate-emarginate ; marginal 

 cells less deeply incised, divisions emarginate, bidentate or sub- 

 truncate. Greenland. Europe. 



