Key to the Species 



1. Trichomes short, living within the mucilage 



of other Cyanophyta (e.g. Microcystis) P. mucicola 



1. Trichomes long, forming plant masses, not 



Uving within the mucilage of other algae 2 



2. Trichomes with constrictions at the cross walls 3 



2. Trichomes without constrictions at the cross walls 6 



3. Trichomes capitate at the apex F- lucidum 



3. Trichomes not capitate 4 



4. Apical cell decidedly tapering and elongate P. tenue 



4. Apical cell not elongate, not or slightly tapering 5 



5. Cells 2.5m in diameter, 2-4m long P. minnesotense 



5. Cells 4-6m in diameter, 1.2-2.7m long P. ambiguum 



6. Trichomes decidedly twisted or hooked at the apex P. uncinatum 



6. Trichomes not twisted, straight or slightly curved at the apex 7 



7. Apical cell capitate ° 



7. Apical cell not capitate , ,,; • 



8. Trichomes with a conical, sharply pointed calyptra P. Setchellianum 



8. Trichomes with a broadly rounded or convex calyptra 9 



9. Trichomes straight 10 



9. Trichomes curved, flexuous P- autumnale 



10. Cells longer than wide P- favosum 



10. Cells shorter than wide P- subfuscum 



11. Plant mass incrusted with lime P- incrustatum 



11. Plant mass not incrusted with lime 12 



12. Cross walls granular P. inundatum 



12. Cross walls not granular 13 



13. Trichomes curved; cells 3-4.5u in diameter, 3.4-8m long P. Corium 

 13. Trichomes straight; cells 4.5-12 u in diameter, 



4-9m long P- Re*Z" 



Phormidium ambiguum Gomont 1892a, p. 178 

 PL 111, Fig. 1 



Filaments forming a blue-green, mucilaginous layer; straight or 

 gracefully curved, either parallel or somewhat entwined; individual 

 sheaths usually distinct and lamellate but becoming confluent with 

 the mucilage of the plant mass; trichomes curved or rarely straight 

 at the apices, which are not tapering; apical cell broadly rounded, 

 not capitate, but with a thickened outer membrane, which may 

 give a slightly pointed appearance; cells short, disc-like constricted 

 at the cross walls, 4-6/^ in diameter, 1.2-2.7/x long; cell contents 

 finely granular. 



Among other algae and floating free in tychoplankton. Wis. 



Phormidium autumnale (C. A. Ag.) Gomont 1893, p. 207 



PI. 107, Figs. 19, 20 

 Plant mass forming a broadly expanded, dark-green, mucilaginous 

 layer; filaments much entangled but may be either straight or 

 curved and flexuous; sheaths at first distinct, becoming diffluent 



[493] 



