WISCONSIN PHYTOPLANKTON 21 



EUASTRUM Ehrenberg 1832 (emend. Ralfs 1844). 



Cells of variable size, length generally about twice the breadth, 

 deeply constricted, sinus generally linear and with apex inflated and 

 outer portion open ; semicells usually truncate-pyramidate, incised to 

 form three or five lobes, polar lobe always with an apical incision 

 which is usually deep but which may be reduced to a shallow depres- 

 sion ; incisions forming lateral lobes frequently greatly reduced. Cell 

 wall smooth, punctate or granulate, sometimes with spines on outer 

 margin; with one or more protuberances at base of semicells and at 

 times with other protuberances at center of cell face or at poles; in 

 certain species with small rounded or elongate verrucae definitely 

 arranged on the cell wall. Vertical view generally elliptic, with poles 

 rounded and definite protuberances at sides. Lateral view of semi- 

 cells narrowly truncate-pyramidate, with basal inflations fairly prom- 

 inent. Chloroplasts of larger species axial and with two lobes to each 

 cell face that end in massive parietal laminae ; chloroplasts of smaller 

 species axial and with simple lobes; in one case with two chloroplasts 

 in each semicell; pyrenoids single to numerous. 



Zygospores spherical to ellipsoidal; with smooth, mamillate or 

 short-spined walls. 



The compressed cells in which the length is not more than about 

 twice the breadth show that this genus is related to Cosmarium, but 

 the apical incision distinguishes it from Cosmarium. This apical 

 incision may be reduced to a shallow depression so that the distinction 

 between the two genera is very finely drawn. The apical incision is, 

 however, associated with the presence of one or more protuberances 

 at the base of the semicells. In the apical incision it shows an affinity 

 with certain elongate genera, as Tetmemorus and Ichthyocercus, but 

 in these genera the cells are not conspicuously compressed and the 

 length is several times the breadth. 



KEY TO THE SPECIES. 

 Polar lobe incised. 



Semicell base with more than one protuberance. 



Three-lobed semicells (1) -E. crassum 



Five-lobed semicells. 



Polar lobe rectangular (2) E. didelta 



Polar lobe not rectangular. 



Protuberances of the same size (3) E. oblongum 



Protuberances not of the same size. 



Two small protuberances above isthmus (4) E. affine 



One large protuberance above isthmus (5) E. pinnatum 



Semicell base with one protuberance. 



Polar incision ovate (6) ^. Glaviovii 



Polar incision not ovate. 



Apex flattened (7) E. pulchellwm 



Apex elevated. 



Lateral lobes with spines (9) E. divaricatum 



Lateral lobes without spines (8) E. elegmis 



Polar lobe emarginate. 



Lateral lobes cuneate (110 ^- verrueosurtf 



Lateral lobes subquadrate (10) E. gemmatun^ 



