6 ^ WlySCONSIN PHYTOPLANKTON 



The investigations of Liitkemiiller show that the segmentation or 

 lack of segmentation of the wall, a feature that is associated with the 

 presence or absence of pores, is a fundamental point in the classifica- 

 tion of the family. Most genera belong to the first type ; Gonatozygon, 

 Spirotaenia, and Netrium are the only genera with unsegmented walls 

 found in the Wisconsin plankton. 



Spirotaenia condensata De Brebisson. PL 52, Fig. 4. 



in Kalfs, Brit. Desm. 179, pi. 34, fifjs. lA-lG. 1848; Archer, Quart, Jour. Micr. 

 Sci. N.S. 7: 186, pi. S, figs. 5-11. 1867. 



Cells relatively large, straight or slightly curved, length 5-12 times 

 the breadth ; cell shape cylindrical, not attenuated towards the apices, 

 poles broadly rounded. Cell Avail smooth. Chloroplast a broad spiral 

 ribbon, extending from pole to pole and almost completely covering 

 the interior of the cell wall; pyreuoids fairly numerous^ at least one 

 every turn. /^Tychoplanktont.) 



Zygospores spherical; wall with a thin, inner, brownjsh layer and 

 a thick, colorless, areolate, outer laj'er (Archer). 



Length 150 //, ; breadth 2.5 /x. 



Diameter zygospores 60//, (Archer). 



Devils (rrr). 



NETRIUM Nageli 1840 (emend. Liitkemiiller 1902). 



Cells generally large, length at least three times the breadth, 

 neither constricted in the median portion nor compressed; cell shape 

 cylindrical to naviculoid, with lateral margins gradually attenuated 

 to truncate apices, or parallel and then sharply convergent in the 

 apical region. Cell wall always smooth and without pores, Chloro- 

 plasts axial, usually one in each half-cell, with 6-12 longitudinal, 

 radiating plates that have strongly notched edges; pj^renoids always 

 axial, usually a single long rod, but at times with the rod fragment- 

 ing to form an axial row of spherical to irregularly shaped pjn-enoids. 

 Cell division at no definite point. 



Zygospores spherical, with smooth, thick walls. 



W. & G. S. West (Monogr. Brit. Desm. 1: 63) give the reasons for 



adopting Liitkemiiller 's separation of Netrium from Penium on the 

 basis of the structure of the cell wall. 



Netrium digitus (Ehrenberg) Itzigsohn & Rothe. PI. 52, Fig. 5. 



in Eabenhorst, Die Algeii Saehsens Dee. 50: No. .50S. 18.56; W. & G. S. West, 

 Monogr Brit. Dessn, 1: 64, pi. 6, figs. 14-16. 1904; Carter, Ann. of Bot. 33: 227, 

 pi. 14, figs. 13-1.5. 1919. 



Cells usually large, length 8-5 times the breadth, wejtber con- 

 stricted nor compressed ; cell shape broadly naviculoid, with lateral 

 margins convex and attenuated to trmicate apices with rounded 

 angles. Cell wall smooth. Chloroplasts axial, one in each half-cell, 

 usually with eight radiating longitudinal plates that end at the cell 



