Myxophyceae 283 



Genus RIVULARIA (Roth) Agardh. Syst. Algar. 19. 1824. 



Colonies spherical, hemispherical or inflated and lobed, solid or hollow, 

 sometimes confluent into an indefinite mass; filaments radiating from the 

 center, repeatedly branched; sheaths conspicuous near the base of the 

 trichomes, near the periphery of the colony gelatinous and confluent; 

 heterocysts basal; gonidia more or less cylindrical and elongate, not known 

 in all species. 



I Filaments flagelliform, tapering towards the apex; gonidia present 



1 Colonies hard; trichomes 4-7 mic. in diameter; gonidia 9-15 mic. in 



diameter, especially cylindrical R. pisum 



2 Colonies soft; trichomes 7-9 mic. in diameter; gonidia 10-18 mic. in 



diameter, larger at the base R. natans 



3 Colonies firm, solid, light green; gonidia cylindrical, ^frequently 



curved, about nine times as long as broad R. incrustata 



4 Colonies soft, solid; trichomes 8-10 mic. broad at the base; gonidia 



8-18 mic. in diameter, cylindrical, straight or slightly curved 



R. echinulata 



II Filaments gradually tapering; gonidia unknown 



1 Colonies hollow when old 



(i) Colonies soft; trichomes 4-5 mic. in diameter in lower portion, 



somewhat constricted at joints R. polyotis 



(2) Colonies soft; trichomes 2-5 mic. in diameter, cylindrical 



R. nitida 



2 Colonies solid 



(i) Colonies not encrusted with calcium carbonate 

 A Plants living in fresh water 



a Trichomes 4 mic. in diameter R. borealis 



b Trichomes 6-10 mic. in diameter R. compacta 



c Trichomes 9-12.5 mic. in diameter R. minutula 



d Trichomes continuous or indistinctly divided; heterocysts IQ-12 

 mic. in diameter R. paradoxa 



B Plants living in salt water; trichomes 2.5-5 ™ic. in diameter 



R. atra 

 (2) Colonies encrusted with calcium carbonate 

 A Colonies hemispherical, finally confluent and forming a hard, 

 stony crust; trichomes 4-7.5 mic. in diameter 



R. haematites 

 D Colonies small, somewhat hard; trichomes 4-9 mic. m diameter 



R. dura 

 C Colonies at first hemispherical, afterwards forming a gelatinous 

 crust, indurated with calcium carbonate in the interior; trich- 

 omes 5-9 mic. in diameter R. coadunata 

 D Colonies spherical, hard; trichomes 4-16 mic. in diameter 



R. bornetiana 



