WISCONSIN PHYTOPLANKTON 45 



6. MiCRASTERiAS DENTicuLATA De Brebisson. PL 59, Fig. 5. 



Mem. de la Soc. Acad. Falaise 1835: 54, pi. 8. 1835; Ealfs, Brit. Desm. 70, pi. 

 figs. lA-lG. 1848; W. & G. S. West, Monogr. Brit. Desm. 2: 105, pi. 49, figs. 

 1-7; pi. 50, figs. 1-2. 1905; Carter, Ann. of Bot. 33: 299, pi. 19, figs. 2-8. 1919. 



Cells large, broadly elliptic in outline, length somewhat greater 

 than the breadth, very deeply constricted, sinus linear from apex to 

 extremity, isthmus narrow; semicells five-lobed, incisions forming 

 polar and lateral lobes deep and linear; polar lobe with lateral mar- 

 gins of basal portion subparallel and sides of upper portion some- 

 what divergent, apical margin rounded and not exserted, with a 

 broad emargination in the median portion, angles of polar lobe blunt 

 and rounded ; lateral lobes equal in size, cuneate in shape and re- 

 divided by two series of linear incisions, the depth of each succeeding 

 incision being about half that of the preceding series, ultimate lobules 

 with broad truncate ends that have rounded angles and a median 

 emargination of variable depth. Cell wall punctate but without 

 ornamentation. Vertical view narrowly elliptic, with poles forming 

 sharp points and median portion of lateral margins with three small 

 rounded elevations. Lateral view of semicells narrowly pyramidate, 

 with the bases bearing an acutely pointed swelling and apex rec- 

 tangular. Chloroplasts completely filling the cells, portion extending 

 into the polar lobe with a conspicuous excavation, ridges of chloro- 

 plasts following the major incisions and not extending across the body 

 of the semicell; pyrenoids numerous, 20-40 in each semicell. (Tycho- 

 planktont. ) 



Zygospores spherical, with a thick lamellated wall bearing stout 

 spines that are the same diameter throughout and have their apices 

 bifid to trifid. at times with the incised portion recurved (Ralfs). 



Cells 236-275 /A long; 195-235 /x broad at base of semicells, 54-68 /x 

 at apex of polar lobes ; 28 /x broad at isthmus. 



Zygospores 80-116 /a in diameter without spines, 170-195 /x with 

 spines (W. & G. S. West). 



Beaverdam (rrr), Bass (rrr), Speese (rrr). 



7. MiCRASTERiAS RADiosA Ralfs uou (Lyugbyc) Agardh. 



Brit. Desm. 72, pi. 8, fig. 3. 1848, 



Micrasterias Sol (Ehrenberg) Kiitzing, Species Algarum 171. 1849; W. & G. S. 

 West, Monogr. Brit. Desm. 2: 95, pi. 46, figs. 1-2. 1905. 



Cells large, breadth equal to the length, circular in outline, very 

 deeply constricted, sinus linear, with sides undulate and apex acute, 

 isthmus very narrow; semicells five-lobed, incisions between lobes 

 linear and deep ; polar lobe arising near the isthmus, with lateral 

 margins subparallel and apical portion somewhat expanded, apical 

 margin deeply emarginate and with lateral margins deeply bifid, 

 emargination flanked on either side by a subapical mamillate projec- 

 tion that bears a single conspicuous erect spine ; lateral margins deeply 

 incised, with inner portion of incisions more open and giving the 

 lobelets an undulate appearance; superior lateral lobe usually some- 

 what larger than the inferior, superior of lateral lobes with three 

 series of incisions the depth of each successive incision a third less 



