THE GREEN ALGAE OF NORTH AMERICA 177 



4. Lobes of marginal cells incurved, forcipate. 3. P. f rcipatuin. 



4. Lobes of marginal cells straight or nearly so. 5. 



5. Margin finely tuberculate-crenulate. 5. /*. vagum. 



5. Margin not tuberculate-crenulate. 6. 



6. Lobes ending in short, broadly triangular teeth. 



6. P. angulosum. 



6. Lobes ending in linear teeth. 4. P. Boryanum. 



7. Disk continuous. 8. P. tetras. 



7. Disk perforate. 9. P. biradiatum. 



8. Marginal cells with a single tooth or cusp. i. P. simplex. 



8. Marginal cells tridentate. 2. P. tricornutum. 



1. P. SIMPLEX Meyen, 1829, p. 772, PI. XLJII, figs. 1-5 ; 

 Wolle, 1892, p. 168, PI. LrXIV, fig. 17; Wittr. and Nordst., 

 Alg. Exsicc., No. 524. Family of 8-16, rarely 32 cells, up to 

 75 p. diam.; arrangement much varied, disk continuous or per- 

 forate ; each marginal cell ending in a single aculeate tooth or 

 spine. 



Var. STURMII (Reinsch) Wolle, 1892, p. 168, PI. L,XIV, 

 fig. 1 8. Cells arranged in one or two series about a large cen- 

 tral open space. 



Var. DUODENARIUM (Bailey) Rabenhorst, 1868, p. 71 ; Wolle, 

 1892, p. 169, PI. LXIV, fig. 20. Disk with a central open 

 space, surrounded by four cells, four openings in the form of 

 arcs of circles between these cells and the 12 cells of the outer 

 row. Mass., N. Y., Ohio, 111. Europe, So. America. 



These varieties are hardly worthy to be distinguished by 

 name, being merely two of the many forms assumed by this 

 species. None of the forms, however, need be mistaken for any 

 other species, as there is no other species in which each cell 

 ends in a long, aculeate projection. 



2. P. TRICORNUTUM Borge, 1892, p. 4, fig. 3. Disk contin- 

 uous, rounded, 32-40^ diam.; marginal cells 10-18X9-10 ^, tra- 

 peziform, with three teeth on the margin ; disk cells polygonal. 

 Greenland. Northern Europe. 



3. P. FORCIPATUM (Corda) A. Braun, 1855, p. 86; Wolle, 

 1892, p. 169, PI. L/XIV, figs. 21, 30-31. Cells 8-16, rarely 

 more, forming a continuous disk ; disk cells about 24 /A wide, 

 polygonal or slightly incurved on the outer side ; marginal cells 

 deeply incised, bilobed, lobes incurved, acuminate. Mass., 

 N. J., Pa. Europe. 



The small disks, and especially the forcipate prolongations of 

 the marginal cells, seem sufficiently characteristic. 



4. P. BORYANUM (Turp.) Meneghini, 1840, p. 210; Nageli, 

 1848, p. 95, PI. V.B, fig. i ; Wolle, 1892, p. 169, PL L,XIV, 



