266 FRESH-WATER ALGJE OF THE UNITED STATES. 



lowish ; sheath very thin, colorless or yellowish ; heterocysts 

 usually single, spherical or somewhat longer than broad. 



Diameter of filaments, 3-4 //. 



Ponds, Pennsylvania and New Jersey, often very 



abundant. 



i 



Plate CLXXXI, figs. 5-7, three parts of filaments. 



TOLYPOTHRIX TRUNCICOLA, ( Bab. ) Wolle. 



In small caespitose clusters, usually confluent, surface 

 velvet-like, fuscous black ; trichomes ascending, fascicu- 

 late, concreted below, internally dull aeruginous or olive 

 yellow ; articulations granular, short, about one-third as 

 long as broad ; sheath close, colorless or light yellow, indis- 

 tinctly lamellate; heterocysts, subspherical, singly at the 

 base of the branchlets. 



Diameter of filaments, 11-14 ^JL. 



Frequent on trunks of trees, South Carolina. 



Syn. Scytonema truncicola, Rab. 



In adopting this name it may be proper to state that 

 Rabenhorst in his diagnosis does not allude to heterocysts, 

 hence may be a different plant ; beside the omission of this 

 feature the agreement is sufficiently close. 



Plate CLXXXI, figs. 8-11, from trunks of trees. 



Genus 89, PLECTONEMA, Thur. 



Filaments branched, singly or geminate, rather irregular ; 

 every filament enclosed in a distinct sheath ; devoid of hetero- 

 cysts ; cell contents deep aeruginous. 



PLECTONEMA MIRABILE, Thur. 



Filaments usually deep aeruginous ; older plants change 

 to a shade of brown ; forms dense dark green tuft, ]-* inch 

 in length on aquatic plants in limestone springs. Articu- 

 lations shorter than their diameter ; granular, sheath nar 

 row, colorless or yellowish, smooth. 



Diameter of filaments, 15-20 ju. 



Syn. Conferva mirabilis, Dillw. ; Calothrix Brebissonii, Kg. ; Calo- 

 thrix atrovirens, Harv. ; Calothrix mirabilis, Ag. ; Scyto- 

 nema Naegelii, ( Kg. ) Wood. 



Limestone springs, attached to submerged plants, wood 



and stones. 



\ 



Plate CLXXXI, figs. 12-15, four type-forms of branching, 

 inclining discs, short cells, etc. 



