WISCONSIN PHYTOPLANKTON 135 



species that cannot be confused with any other of the genus Arthro- 

 desmus being the only one with trapeziform semicells that bears long 

 spines at the angles. 



The alga occurred in sufficiently quantity in Travis Lake to war- 

 rant the assumption that in this case it was multiplying rapidly under 

 plankton conditions. 



ONYCHONEMA Wallich 1860. 



Cells generally small, breadth usually somewhat greater than the 

 length. United to form permanent filaments by two apical processes 

 that lie on opposite sides of the apex and overlap adjacent cells of 

 the filament. Cells compressed, deeply constricted and generally with 

 a closed sinus; semicells elliptic, with a smooth or serrate outline, 

 lateral angles smooth or furnished with a single long convergent 

 spine. Cell wall smooth or with transverse rows or granules or pores, 

 at times finely punctate. Vertical view fusiform-elliptic, with the 

 apical processes of the front view alternately arranged and the poles 

 rounded or bearing a single straight spine. Lateral view of semicells 

 usually circular. Chloroplasts axial and with one pyrenoid. Colonies 

 generally enclosed in a fairly broad gelatinous sheath. 



Zygospores spherical and ornamented with short spines. 



The genus is closely related to Sphaerozosma Corda but differs in 

 the nature of the apical processes. In Onychonema these processes 

 hold the cells together in a filament by overlapping adjacent cells 

 while in Sphaerozosma the processes merely touch corresponding 

 processes of adjacent cells. 



KEY TO THE SPECIES. 



Lateral angles without spines (1) 0. fiUforme 



Lateral angles vrith spines (2) 0. laeve 



1. Onychonema filiforme (Ehrenberg) Roy & Bisset. PL 86, 



Fiffs. 13-14. 



*&*• 



Jour, of Bot. 24: 242. 1886; Liitkemiiller, Beitr. z. Biol. d. Pflanzen 8: 365, pi. 

 18, fig. 28. 1902. 



Cells small, breadth slightly greater than the length, deeply con- 

 stricted, sinus sublinear, with the apex acute and the outer margin 

 widely open, isthmus narrow ; semicells broadly elliptic, dorsal mar- 

 gin more rounded than the ventral, lateral angles broadly rounded 

 and without spines, apical processes quite long, divergent, and with a 

 very slight swelling at the ends; adjacent cells of the filament not in 

 contact except by the processes. Lateral view of semicells subcircu- 

 lar. Cell outline smooth and wall with two transverse rows of pores. 

 Chloroplasts axial and with one pyrenoid. (Tychoplanktont.) 



Zygospores unknown. 



