FRESH-WATER ALG.E OP THE UNITED STATES. HQ 



(Docidiuvi Ehrenbergii. Ralfs.) Frond slender, linear; suture forming a very sbarply 

 defined rim; segments 8-12 times longer than broad, basal inflation having another suialler 

 one above it, sides otherwise straight, parallel; ends crenate, owing to a number of emar"-iiia- 

 tions on the edge of the truncate extremities, from three to five of the crenations being 

 usually visible; e. f. punctate or rough with minute granules. Sporangium suborbicular or 

 elliptic, or slightly angular, smooth, placed between the deciduous empty fronds. Ciliated 

 zoospores formed by segmentation of the cell contents, and their emission effected through 

 the opened apex of each one, two or three, especially, formed lateral tubes arising from be- 

 -neath the base of one of the segments. Archer. 



Remarlcs. — This species is quite common around Philadelphia ; but I do not 

 remember ever to have seen one with the cell-wall granulate. The smaller of the 

 two umbonations near the centre is often wanting or exceedingly small, and the 

 crenulations in the ends are very often obsolete. 



Fig. 9, pi. 12, represents a cell of this species magnified 160 diameters. 



P. Baciiluiu, (Breb.) Pe Bary. 



P. priori simile, sed gracilius, angustius et plerumque longius, medio tantum serael constrictum • 



cytiodermate laavi. (R.) Species mihiignota. 

 Djam.— 0.00054"— O.OOOy". (R.) 



Syn. — P.Baculum, {^KEB.) DeBary. Rabenhorst, Flora Europ. Algarum, Sect. III. p. 141 



Hab. — Georgia ; Bailey. 



Frond slender, suture not prominent; segments very many times longer than broad, basal in- 

 flation very conspicuous, solitary, sides otherwise straight, very nearly parallel, large gran- 

 ules of the endochrome in a single series; ends entire; e. f. without puncta. L. -rjy"- B. 



1 " 



p. breve, Wood. 



P. robustum, diametro 4-8 plo longius, in medio distincte constrictum sed baud undulatuin, 

 utroque polo nonnlhil attenuatum ; apicibus truncatis et nonnihil rotundatis ; cytiodermate 

 crassissimo, dense granulato-punctato; marginibus vel rectis, vel breve undulatis. 



i)M(m.— .00.38"— .00095". 



Syn. — P. breve, Wood. Proc. Acad. Xat. Sciences, 1809. 



Hab. — District of Columbia. (Billings.) 



Robust, 4-8 times longer than broad, distinctly constricted but not undulated in the middle, 

 slightly attenuated towards the ends; apex truncate and somewhat rounded ; cytioderm very 

 thick, densely minutely granulate ; margins either straight or shortly undulate. 



Remarhs. — This species was sent to me by Dr. Billings, who obtained it near 

 Washington, D. C. The margins are sometimes straightish, but in other fronds there 

 are three or more distinct short undulations, or rounded projections in each half 

 margin. The cell-wall is excessively thick, especially at the end — in many cases 

 much thicker than the drawing. 



Fig. 2, pi. 21, represents an empty frond of this plant magnified 750 diameters. 



P. crcniilntiiiii, (Ehrb.) Rabenhorst. 



P. robustum, cylindraceo-subclavatum, octies-duodecies longius quam latum, medio undulato- 

 nodulosum, strictura3 mediae margine tumido, apicibus late truncatis, altero sajpe creuulalo ; 

 cytiodermate granulato-punctato. (R.) Species mihi ignota. 



Diam.— 0.0023". (R.) 



Syn. — P. crenulatum, (Ehrb.) Rabenhorst, Flora Europ. Algarum, Sect. III. p. 142. 



