28 FRESH-WATER ALG^ OP THE UNITED STATES. 



provided with distinct broad brownish sheaths : every gradation exists between 

 these forms and the first described fronds. The heterocysts are quite uniform in 

 size, agreeing in diameter with the largest vegetative cells, they are always single., 

 This species is most nearly allied to N. ichthyoon, Rabenh. ; from which it is 

 separated by the differences in the sheaths, the greater size of the filaments, and 

 the single heterocysts. It gives me great pleasure to dedicate the species to Mr. 

 Austin, by whom it was collected near Gloucester, New Jersey, growing amidst 

 mosses on rocks. 



W. pruniforme, (Roth,) Agh. 



N. magnum, gregarium, noncohserens, globosum, magnitadine pisi, pruni majoris et ultra, oliva- 

 ceum vel saturate aerugineum, astate provecta fusco-nigrescens, haud raro cavum, Isevissimum, 

 intus aquosum, peridermate coriaceo subachrco ; trichomatibus subaequalibus, hie illic tumidis, 

 laxe intricatis; articulis globosis, plerumque compressis, saepe didymis, arete connexis; eel- 

 Inlis perdurantibas artieulis duplo majoribus, plerumque terminalibus, rarius interjectis. R. 

 Species mihi ignota. 



Dmwi.— Artie. 0.00024"— 0.0003" ; cell, perdur. 0.0003—0.00045". (R.) 



Syn. — N. pruniforme, (Roth,) Ag. Rabenhorst, Flora Europ. Algarum, Sect. II. p. 168. 



Hab. — Maine. Leidy. 



Large, gregarious, not cohering, globose, varying from the size of a pea to a large plum, or 

 even beyond this, olivaceous or deep asrugineous, in old age blackish fuseous, often hollow, 

 very smooth, within watery, periderm coriaceous, somewhat transparent; filaments subequal, 

 here and there swollen, laxly intricate ; articles globose, mostly compressed, often twofold, 

 closely connected ; heterocysts twice the size of the vegetative cells, mostly terminal, rarely 

 interspersed. 



Bemarlcs. — 1 have never found this species ; but some years since some speci- 

 mens, sent to the Academy of Natural Sciences of Philadelphia from Maine, were 

 identified by Professor Joseph Leidy as belonging to it. 



Hr. Terrucosnm, (Linn.) Vatjch. 



N. magnum, gregarium, bipollieare et ultra, subglobosum, ssepe lobatum, verruculosum, irrora- 

 tum, initio solidum, postremo eavum, vesiciforme, saturate brunneo-viride ; peridermate mem- 

 branaceo-coriaeeo, olivaceo-fuscescente; trichomatibus varie curvatis, centralibus parcioribus 

 et laxissime intricatis, periphericis densius intricatis; artieulis oblongis, rare globosis, arete 

 connexis, crasse granulatis ; cellulis perdurantibus interstitialibus vel terminalibus, sphaericis, 

 articulorum diametro duplo majoribus. 



Diam.— Cell, vegetativ. .000166"; cell, perdurant. .000233". 



Syn.—N. verrucosum, (Linn.) Vatjch. Rabenhokst, Flora Europ. Algarum, Sect. II. p. 176. 



Hah. — In fonte. Centre County, Pennsylvania. 



Large, subglobose, often lobed, warty; gregarious, two inches in diameter, growing under water, 

 fixed, in the beginning solid, afterwards hollow, bladder-shaped; periderm membranaceous, 

 coriaceous, olivaceous-fnscons; filaments variously curved, centrally fewer, and laxly intricate, 

 towards the outside much more close; articles oblong, rarely globose, closely connected, 

 coarsely granulate; heterocysts interstitial or terminal, spherical, twice the size of the other 

 joints. 



Remarhs. —Irv the summer of 1869, 1 found a nostoc growing in great abundance 

 in a very cold, large, limestone spring in Centre County, Pennsylvania, which I 



