178 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



figs. 29-32; P. B.-A., No. 1180. Cells 4-64, rarely 128, 10-20 ^ 

 wide, forming a continuous, circular or elliptical disk ; disk 

 cells 4-6-angled, the external side varying from prominent to 

 repand ; marginal cells more or less emarginate or bilobed, each 

 lobe ending in a longer or shorter terete, obtuse to capitellate 

 projection. Fig. 63. Greenland, Me., Mass., N. J., Pa., Ohio, 

 111., Neb., Cal., Alaska. Europe, Asia, So. America. 



A very common and widely distributed species, with many 

 varieties ; to be recognized more by the general combination of 

 characters than by any one detail, though the two terete pro- 

 longations from each marginal cell are usually sufficiently 

 characteristic. 



Var. UNDULATUM Wille, 1879, p. 28 ; P. nndnlatum (Wille) 

 Boldt, 1893, p. 157, fig. i. Cells sometimes 256 ; larger than in 

 the type ; disk cells with irregularly undulate margin ; marginal 

 cells distinctly bilobed. Greenland. Europe. 



Var. GRANULATUM (Kiitz.) A. Braun, 1855, p. 90 ; P. B.-A., 

 No. 1324 ; with smaller cells and families, and verrucose mem- 

 brane. Greenland, Nebraska and California. 



5. P. VAGUM Kiitzing, 1845, p. 143; A. Braun, 1855, p. 82, 

 PL VI, figs. 27-28 ; P. B.-A., No. 1519. Cells 32-128 in a disk, 

 disk continuous, of varying form, up to 250 ^ wide, cells in 

 disk to 30 /j. wide, 5-6-angular ; marginal cells about 10 p. wide, 

 with two short, obtusely triangular projections ; margin and 

 membrane generally finely tubercular-crenulate. Greenland, 

 Me., Mass. Eiirope, So. America. 



The large disk of irregular form, composed of many cells, 

 with the crenulate margin, makes this quite a distinct species. 



6. P. ANGULOSUM (Ehrenb.) Meneghini, 1840, p. 211 ; A. 

 Braun, 1855, p. 84, PI. VI, fig. 26 ; Wolle, 1892, p. 169, PL 

 LXIV, fig. 28. Cells 16-64, U P to 19 /A diam., 5-6-angular, 

 forming a continuous disk, up to 120 /x diam. ; marginal cells 

 with truncate base, wider above, with a more or less deep sinus, 

 the inner side of the ray prolonged in a slightly incurved tooth, 

 the outer edge with a slight apiculum. Me., N. J., Ohio, Neb., 

 Alaska. Europe, Asia. 



Each marginal cell has a projection on each side of the cen- 

 tral rounded sinus ; the inner edge of this terminates in a sub- 

 linear tooth, not exactly on the radial line, but parallel to the 

 tooth on the other side, of the sinus ; the outer edge of the pro- 

 jection has merely a small apiculum, as if produced by the 

 pressure of the adjacent cell. 



