FRESH-WATER ALQ& OF THE UNITED STATES. 95 



bodies arc thus produced out of each original microgonidium. They are large, 

 ovutc, biciliate, and, generally, soon escaping from the hernia! sac, move about 

 actively in the water for a few minutes. Sometimes, however, they settle down 

 within the generative utricle. In either case, after a little time, they become 

 motionless, lose their cilia, and develop into polyhedral cells, which are structurally 

 remarkable for having their angles prolonged into long horn-like appendages. Under 

 favorable circumstances, at the end of a few days, the bright green endochrome of 

 these undergoes similar changes to those described as presaging the production of 

 tin microgonidia, and is finally formed into zoospores, which, in from twenty to forty 

 minutes, unite, within the polyhedron or large cell, into a Hydrodictyon, which is 

 finally set free by a solution of the cellulose coat of the polyhedron. The network 

 thus formed differs in no essential way from that which arises in the better known 

 way, except that it is composed of much fewer cells. It is generally a closed 

 sac ; but when the polyhedron, out of which it is developed, is small, it is some- 

 times merely an open network. Its after-history appears to be identical with that 

 of the ordinary hydrodictyon frond. 



II. utriculatiim, ROTH. 



Species nnica. 



Syn. II. utriculatum, ROTH. RABENHOBST, Flora Europ. Algarnm, Sect III. p. 66. 



Hal, In aquis quietis. West Point, Bailey. Weehawken, (Mr. Walters.) "Waterholes 



between Van Hum's Mills and Mueote on the Mexican boundary, Dr. Bigelow. Pennsyl- 

 vania, Wood. 



Genus PEDIASTRUM, MEYEN, (1829.) 



Ccenobinm plannm, disciforme, libere natans, e cellaarnm strato nnico, rarius centre entro duplicate, 

 continue vel perforate formatum. Cellnlas polygoniw, periphericB saepe bilobas, lobis cuneatis et 

 gimplicibas et bidentatis, nonnunqnam in cornna prodactis. 



Casnobium plain, discoid, swimming free, formed of cells in a single, rarely in the centre double 

 stratum, which is continuous or perforate ; cells polygonal, the peripheral often bilobed, the lobes 

 cuneate, either simple or bidentate, sometimes produced into a horn. 



Remarks. The coenobium or cell-family, or colony, in the genus Pediastrum is 

 always discoid, and has generally a more or less truly circular outline. The cells 

 are mostly in a single stratum, but in some species there are two, more or less, 

 complete strata superimposed one upon the other. The arrangement of the cells 

 in this stratum, or these strata, varies greatly, as does also their number. They 

 arr mostly more or less polyhedral, and often have their margins scooped out or 

 their angles prolonged. This may occur in such a way that the projecting point 

 of one cell fits into the hollow in its neighbor, and the ccenobium be rendered 

 entire, or, no such relation existing between the parts of adjacent cells, the cceno- 

 bium mjiy be perforated with regular or irregular openings. The outer or mar- 

 ginal cells are often deeply notched externally, and frequently are prolonged into 

 acute or obtuse lobe-like processes. The walls of the cells are, in adult specimens, 

 quite thick. The contents consist of chlorophyl, protoplasm, starch granules, &c. 



