54 
GEOLOGY pP THE FIRST DISTRICT. 
have lately received from being detected in a fossil state in Europe and America, will be suffi¬ 
cient inducement for all lovers of microscopic research to study this family, 
I shall now proceed to describe the most interesting American species, commencing with 
Section I. DESMIDIACEA. 
Eesmidium. Free, carapace simple, urceolate, trilateral, often catenate. Mandl and Ehrenberg, 
1. c. p. 244.) 
Desmidium schwartzii, Agardh. (PI. 21, fig. 1.) Corpuscles smooth, quadrangular on three sides, 
slightly emarginate, triangular on the other two sides, end ohtuse, ovarium green, to of 
Paris line. 
The corpuscles of this species are united together by their triangular 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 water which is not distinguishable from a mass of some species of 
Zygnema, with which remarkable genus of Algm 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 Europe, 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, where I have found it most abundant in 
early spring, 1 believe Desmidium cylindricum of Greville to be merely a state of this species, 
Desmidium hexaceros, Ehrenherg. (Compare figs. 2 and 3, PI. 21.) Corpuscles binary, trilateral, the 
points drawn out to three horns and truncate at the extremity, Jg of line. 
Desmidium aculeatum, Ehr. (Compare figs. 4, 5, and 6, Pi. 21.) Corpuscles spiny trilateral, the points 
drawn out to three truncate horns, often terminating in three spines. 
1 copy Ehrenberg’s description of the two last species, that they may be compared with the 
figures referred to above, which represent various binary triangular bodies, some of which 
agree pretty well with the above characters. They, however, are so unlike D. schwartzii, and 
present so many points of resemblance to Euastrum, that I shall describe them as species of 
that genus. 
Staurastrum. Free, a simple univalve quadrangular carapace. 
Staurastrum paradoxum. Corpuscles rough, single or binary, four setaceous horns in form of a cross. 
Micrasterias staurastrum, M. tetracera, didicera, tricera. (Kuetzing, Linnsea, Vol. 8, p. 509, PI. 20, 
figs. 83, 84 and 85.) 
See Agardh, Systema Algaium, p. 15 ; and Greville in British Flora, Vol. 5, p. 402. 
