14 FRESH-WATER ALG^ OF THE UNITED STATES. 



Hah. — In aquis stagnis, prope Philadelphia. 



Thallus microscopic, subglobose or irregular, floating, aggregated in great numbers; cells glo- 

 bose or subglobose ; cytioplasm finely granulate, pale sruginous green. 



Bemarks. — I found this beautiful little plant forming a dense scum on a stag- 

 nant brick-pond, below the city, in the month of July. The scum was of the 

 " color of pea-soup," and so thick was it, that I think a quart of the plants might 

 have been readily gathered. The fronds were of various sizes, and many of them 

 were apparently undergoing division — some of them seemed to have little fronds 

 in their interior. They were composed of an exceedingly transparent firm jelly, in 

 which the cells were placed, often so as to leave the central parts of the frond 

 empty, merely, forming a sort of filament-like layer around the edge. Rarely they 

 were in such numbers as to be crowded together over the whole surface of the 

 frond. In some of the younger fronds the cells formed a little ball within the jelly, 

 instead of being scattered through its outer portion. I have seen some large single 

 cells three or four times the size of the ordinary frond cell, swimming amongst the 

 plants, of which they are apparently the reproductive gonidia. Their cell-coats 

 are very firm and thick. The fronds themselves are often closely aggregated 

 together into little masses, and I think it probable that there is a state of the 

 plant, in which the jelly becomes softened and the fronds more or less fused together 

 in protococcus-like masses. This plant appears to be the same as the European G. 

 duhium, but differs from the description in the fronds not attaining to anything 

 like the size. It is very probable, however, that this depends upon age or circum- 

 stances of growth, and that American plants may be found as large as the 

 European. 



Genus MERISMOPEDIA, Meten. 



Cellulaj globosse, aut oblongse, aut ovales, tegumentis confluentibus, 4, 8, 16, 32, 64, 128 in fami- 

 lias tabulatas, unistratas consociatae. Thallus planus, tenuis, plus minus quadratus, in aqua libera 

 natans. Cellularum divisio in planitiei ntramque directionem. 



Cells globose, oblong, or oval, joined together by their confluent coats into tabular families of 4, 

 8, 16, 32, 64, 128. Thallus, a more or less quadrate plane, swimming free in the water. Division 

 of the cells occurring in all directions in the one plane. 



M. nova, Wood. 



M. thallo membranaceo, distincte limitato, cellulis numerosissimis composito ; cellalis ovalibus, 

 arete approximatis, 16 in familias consociatis, dilute cssruleo-viridibus, interdum medio con- 

 strictis; thalli marginibus rectis, integris. 



Syn.—M. nova, Wood, Prodromus, Proc. Amer. Philos. Soc, 1869, 123. 



Diam.— Cell. ad. jxrW = 0.0025". 



Hab. — In flumiue Schuylkill, prope Philadelphia. 



Thallus membranaceous, distinctly limited, composed of very numerous cells; cells oval, closely 

 approximated, consociate in families of 16, light bluish-gree.n, sometimes constricted in the 

 middle ; margin of the thallus straight and entire. 



BemarJcs— The only specimens I have ever seen of this species were found grow- 

 ing in the Schuylkill River adherent to, or entangled in, a lot of filamentous algse. 



