FRESn-WATER ALGiE OF THE UNITED STATES. 15 



The frond is very sharply defined, and, under a low power, is of a uniform bluish- 

 green tint. The cells are associated in primary families of 16, of a number of 

 which the thallus is composed. The species appears to be most closely allied to 

 M. mediterranean Naeg., from which it differs very essentially in the size of the fronds, 

 and perhaps even more closely to M. glauca, the only character separating it from 

 which is the straight margin. I have myself some doubts whether it ought not to 

 be considered as merely a form of M. glauca. 



Fig. 8, pi. 8, represents this species, magnified 400 diameters. 



Itl. conToluta, Breb. 



M. thallo membranaceo, oculis nudis visibili, plus minus convolute; familiis c cellulis gcuiinis et 

 in subfamilias dispositis, 256 compositis, interdum familiis duabus in familia guraiua conjunc- 

 tis; cellulis spbaericis aut oblongis; cytioplasmate homogeneo, viridi. 



Diam.—Cd\. ^ttW =0.00017"; fam. long. ^\^" = .00"; lat. ^\^" = 0.01". 



Eah. — In aquis quietis prope Philadelphia. 



Thallus membranous, visible to the naked eye, more or less folded; families composed of 25(5 

 geminate cells, arranged in subfamilies, sometimes two of these families conjoined with a com- 

 posite family; cells spherical or oblong; cytioplasm homogeneous, green. 



Remarks. — When my Prodromus was published, the only specimens of this plant 

 which I had seen were contained in a mounted slide given me by my friend Dr. J. 

 Gibbons Hunt, of this city. Since then I have found it growing in a very shallow, 

 quiet, but fresh, sweet pool at Spring Mills, making a distinct green layer upon the 

 mud many feet in extent. Of course, there were millions of specimens in this layer. 

 The fronds are irregular in shape, often somewhat ovate, sometimes subquadrate, 

 variously torn, and not rarely somewhat lobate. Their edges are frequently very 

 sharply defined and rendered firm and prominent by several rows of cells being 

 crowded closely together along them. The cells in the body of the frond are arranged 

 in large parallelogrammatic families, composed of 256 cells. There are 16 cells on 

 each side, the families being parallelogrammatic rather than square, owing to the 

 oblong shape of the cells. This cell family is composed of four subfamilies, each 

 containing 64 cells. These are again subdivisible into four more or less distinct 

 groups of 16 cells each. The cells are, finally, generally closely geminate, each 

 pair being very distinctly separated from its neighbors. In certain stages of growth, 

 as immediately after a general division of the cells, two of the large cell-families 

 spoken of are often temporarily joined together to form a huge family of 512 cells, 

 but soon separate one from the other. 



Order j^ematog'enese. 



Plantse multicellulares vel pseudo-multicellnlarcs. Ccllulre fdum (trichoma) forraantes et ple- 

 rumque vagina tubulosa homogenea vel lamellosa inclusa?. Tricbomata aut simplicia aut ramificata. 



Plants multicellular or pseudo-multicellular. Colls forming a filament, and generally included in 

 a tubular lamellate or homogeneous sheath. Filaments either simple or branched. 



