42 FRESH-WATER ALG.E OF THE UNITED STATES. 



Syn. Sphaerozyga, (AUCTOEES, partim.) 



Dolichospermum, THWAITE'S MSS. Mr. J. RALFS on the Nostochinex, Ann. Mag. 

 Nat. Hist. 1850, p. 335. 



Remarks. This genus differs from Sphcerozyga in that the spores have no rela- 

 tion, in regard to position, with the heterocysts. Professor Rabenhorst, in his 

 Flora, does not acknowledge it ; but it is very evident that he has neither seen 

 the original paper of Mr. Ralfs, nor the species upon which the genus was founded, 

 for he mentions none of the latter, either as good species or synonyms, and the 

 memoir itself is not included in his bibliographical list. The generic characters 

 given by myself are essentially those of the original description, with the excep- 

 tion that the filaments in the latter are said to be aggregated into a stratum, which 

 is not true of the American forms herein described. 



IK snbrigidum, WOOD. 



S. natans; trichomatibus singulis, rectis aut subrectis, minimi's, dilute viridibus; articulis 

 cylindraceis aut subglobosis, distinctis ; sporis cylindraceis, in medio gradatim nonnibil 

 constrictis, singulis ant duplicis, sine cellulis perdurantibus inter se ; cellulis perdurantibus 

 breve cylindraceis, singnlis, distinctis. 



Syn. Sphxroziga subrigidum, WOOD, Prodromns, Proc. Amer. Philos. Soc., 18C9, p. 123. 



Diam. Cell. veg. trans. sAu" = .00016"; spor. transv. ,^5" ?sW = -00023" .00022"; 



lon g- T5W = - 066"; cell. perd. transv. ?5 V/ = .00022". 

 Hab. In stagnis prope Philadelphia. 



S. Floating; filaments single, straight or straightish, very small, light green; articles cylin- 

 drical or subglobose, distinct; spores single or double, in the middle gradually a little con- 

 stricted, not having a heterocyst between them ; heterocysts shortly cylindrical, single, 

 distinct. 



Remarks. I have found this species growing in the scum floating upon the 

 ditches below the city. The filaments are always, as I have seen them, scattered. 

 They seem always to be nearly straight, or entirely so, and indeed preserve their 

 straightness so constantly as to suggest the name given the species. The spores are 

 very distinct, and all that I have seen were greenish, cylindrical, and constricted in 

 the middle, so that their sides are concave. Their position does not seem to be 

 uniform, any further than that they are amongst the ordinary cells. The heterocysts 

 are large, almost equalling the spores in diameter ; I have never detected hairs on 

 them. This species appears to be most nearly allied to D. Thwaitesii of Ralfs, from 

 which it differs in not forming a stratum, and in the great proportionate diameter 

 of the heterocysts. I have never seen any measurements of D. Thwaitesii. 



Fig. 2, pi. 3, is a filament, magnified 975 diameters. 



D. polysperma, (KTZ.) 



S. trichomatibus plernmque snbsolitariis, sed interdum consociatis et intricatis, dilute cseruleo- 



libus, subrectis aut varie cnrvatis et flexuosis ; articulis aut subsptisericis aut breve cylin- 



Incis; cellnlis perdurantibus globosis aut latissime ellipticis, articulorum diametro paulo 



vel dnplo majoribus; sporis pins minus elongatis, cylindraceis in setate immatnra, sparse 



granulatis, dilute caeruleo-viridibus, et cum mernbranahaud distincta, in state matura dense 



granulatis et cum membrana subcrassa. 



