OF THE FAMILY BACILLARIA. 117 



I shall now proceed to describe the most interesting American 

 species, commencing with the 



DESMIDIACEA. 



DESMIDIUM. 



Free^ carapace simple, nrceolate, trilateral,, often catenate. 

 Mandl. and Ehrenberg, I. c. p. 244. 



1. Desmidium Schwartzii. Ag. (PL I, fig. 1.) " Corpuscles smooth, 

 quadrangular on three sides, slightly eraarginate, triangular on the 

 other two sides, end obtuse, ovarium green, one ninety-second to 

 one ninety-sixth of Paris line." 



The corpuscles of this species are united together by their tri- 

 angular faces, so as to produce long triangular (often twisted) 

 filaments, which are of a beautiful green color, and exceedingly 

 lubricous. Each filament is enveloped in a very transparent 

 gelatinous matter, which is not visible on the dead specimens. 

 The filaments often occur together, in great quantities, and form 

 a stratum in the ^vater, which is not distinguishable from a mass 

 of some species of Zygnema, with which remai-kable genus of 

 Algae, this presents many points of resemblance. Allusion is 

 made by several writers* to a " curious pinnatifid appearance " 

 which this species presents " before the ultimate separation of 

 the joints." I have not seen the filaments in this state, nor have 

 I met with any detailed account of this change in the few works 

 which I have been able to consult. 



This species is extensively diffused in Em'ope, and appears to 

 be equally so in this country. I have met with it from Rhode 

 Island to Ouisconsin, and south to Virginia ; it occurs at West 

 Point in great abundance in ditches and peat bogs, ^vhere I have 

 found it most abundant in early spring. I believe Desmidium 

 cylindricum of Greville to be merely a state of this species. 



2. Desmicli%i7nhexaccros, Ekr. (Compare figs. 2 and 3, PL I.) Cor- 

 puscles binaiy, trilateral, the points drawn out to three horns and 

 truncate at the extremity, ^V of line. 



3. Desmidium acideatum, Elir. ( Compare figs. 4, 5, and G, PL I. ) 



* See Agmdh. Systema Algarum, p. 15, and Greville in British Flora, vol. v. p. 40i 



