66 FRESH-WATER ALG^E OF THE UNITED STATES. 



sometimes interspersed, thick-walled, mostly irregularly parallelogrammatic, mostly 4-seriatc, 

 semitransparent, sometimes very sparsely granulate. 



RemarJcs. This species grew spontaneously in the aquarium of my friend Dr. 



Fricke, to whom I am indebted for specimens of it, forming little, bright-green 

 balls adherent to the various aquatic plants. It approaches so very closely the 

 European T. dlstorta, that I have considered it as a mere variety of it, although it 

 differs in having the heterocysts mostly arranged in fours, and also apparently in 

 their shape they being in our plant mostly parallelogrammatic. 



Fig. 1 a, pi. 8, represents a section of heterocysts magnified 800 diameters; fig. 

 1 b, a portion of filament magnified 800 diameters. 



Genus PETALONEMA, BERK. (1833.) 



Scytonematis trichomata vaginis crassissimis e stratis numerossissimis brevioribus, infundibuli- 

 formi dilatatis, imbricatis et plerumque dilutissime coloratis coinpositis. (R.) 



Syn. Arthrosiphon, KTZ. (1845.) 



" Filaments stratified, decumbent, free, simple, or branched. Tube or sheath very wide, flat- 

 tened, longitudinally and transversely striate and crenulate at the edge ; endochrome oliva- 

 ceous annulated, here and there interrupted by a heterocyst. Branches issuing in pairs, 

 formed by the division and protrusion of the endochrome of the original filament. 



" When placed under the microscope the filaments present the appearance of a cylindrical cen- 

 tral column, containing annulated, olive-colored endochrome, and a wide wing-like border at 

 each side of the column. This border or sheath is obliquely striate, the striae running in an 

 arch from the margin toward the centre, where they become parallel, and are then continued 

 longitudinally downward along the medullary column, till lost in the density. The margin 

 of the wing is closely crenulate and in age transversely striate at the crenatures as though 

 jointed. Such is the apparent structure ; the real structure seems to be, that an annu- 

 lated central filament is inclosed within a number of compressed, trumpet-mouthed gelatino- 

 membranaceous tubular sheaths, one arising within the other, and successively developed as 

 the growth proceeds. These sheaths, thus concentrically arranged, are indicated by arching 

 longitudinal striae; and the mouths of the younger sheaths, projecting slightly beyond those 

 of the older, form the crenatures of the margin." HARVEY. 



P. alaliim, BERK. 



A. pulvinato-crustacens, rupicola, varie coloratus; trichomatibus internis serugineis, cnrvatis, 

 parce pseudoramosis, modo continuis, modo torulosis, submoniliformibus, apice plerumque 

 pauluni iucrassatis, saepe roseolis, rotundatis; articulis distinctis, granulosis, diametro sub- 

 sequalibus vel paulo brevioribus; vaginis stratis internis, aureis vel aureo-fuscentibus, 

 externis achrois, vitreis; cellulis perdurantibus interjectis et ad psendoramulorum basin, 

 plerumque solitariis, subglobosis vel oblongis, dilute fuscis. (R.) Species mihi ignota. 



Diam. Trich. intern. 0.00016" 0.00032" ; vag. 0.003T7". (R.) 



Syn. Arthrosiphon alatus, (GREV.) RABENH. Flora Europ. Algarum, Sect. II. p. 265. 



Petalonema alatum, BERKELEY. HARVEY, Nereis Boreis Americana, part iii. p. 99, 

 Smithsonian Contributions, 1846. 



Hob. " On dripping rocks under Biddle Stairs, Niagara Falls." (Harvey.) 



" This forms strata of a dark chestnut-brown color and of indefinite extent on the surface of 

 rocks or soil exposed to the constant drip of water. The filaments are decumbent, lying 

 without order in the gelatinous matrix in which they are developed, and which forms the 



