8 WISCONSIN PHYTOPLANKTON 



2. Penium minutum (Ralfs) Cleve. PI. 52, Fig. 7. 



Of vers. Kgl. Sv. Vet.-Ak. Forh. 20: 493. 1864. 



Docidium minutum Ralfs, Brit. Desm. 158, pi. 26, f,gs. 5A-5B. 1848. 



Cells of medium size, length about ten times the breadth, slightly 

 constricted, sinus a small V-shaped depression, isthmus very broad ; 

 semicells cylindrical, with lateral margins very slightly attenuated 

 towards the apices, bases of semicells not inflated, apices truncate, 

 with angles somewhat rounded. Vertical view circular. Cell wall 

 smooth or finely punctate. Chloroplast axial, with an axial row of 

 about six pyrenoids. ( Tychoplanktont. ) 



Zygospores unknown. 



Length 116)u,; breadth semicell base 10 /n, semicell apex 6.25 ;tt; 

 breadth isthmus 7.5 ju. 



Lindy (rrr). 



The specimens observed were somewhat narrower than the typical 

 P. minutum and approached the variety gracile. The lack of a basal 

 swelling of the semicell, as well as a lack of apical or basal ornamen- 

 tation, shows that the species cannot be referred to either Docidium 

 or Pleurotaenium. 



CLOSTERIUM Nitzsch 1817. 



Cells greatly elongated and always, usually markedly, attenuated; 

 in most cases curved, rarely straight ; without a median constriction. 

 Cell wall colorless or yellow to brown in color ; smooth or with longi- 

 tudinal striae that are usually ridges but may be series of granules; 

 commonly with one or more transverse lines at middle of cell or at 

 different points along the cell. Cells with a single chloroplast in each 

 semicell that is either entire or with longitudinal ridges radiating 

 from a comparatively slender central axis; generally with a single 

 axial row of pyrenoids, more rarely with pyrenoids scattered through- 

 out chloroplast. Cell apices hyaline in portion not occupied by chloro- 

 plasts and with a conspicuous vacuole that contains one or more 

 granules that show a Brownian movement. 



Zygospores spherical, ellipsoid, ovoid, or quadrangular; with a 

 heavy, smooth or scrobiculate wall that is without processes. 



KEY TO THE SPECIES. 



Cells with a median girdle. . (1) CI. subtruncatum 



Cells without a median girdle. 

 Poles not sharply attenuated. 



Curvature pronounced (2) CI. Venus 



Curvature not pronounced. 



Inner margin tumid (3) CI. moniliferum 



Inner margin not tumid (4) CI. aceroaxtm 



