60 
GEOLOGY OF THE FIRST DISTRICT. 
This is a very pretty species, which occurs at West-Point. The figure shows the position 
of two groups of active molecules. 
Arthrodesmus convergens. (Figs. 11 and 12, PI. 21.) Binary, each corpuscle elliptical, and terminating 
at each extremity in a single spine. Hab. West-Point. 
Euastrum -. (Figs. 2, 3, 4, 5, 6, and 7, PI. 21.) Binary, (sometimes quaternary,) generally trian¬ 
gular, and terminating in three long arms, each of which ends in three minute spines. Hab. W est-Point. 
The number of arms is usually three; but I have met with specimens in which one cor¬ 
puscle had three and the other four arms, others in which both had four, and others again in 
which both had five arms. 
It appears to me that the five-armed variety may have given rise to the genus Pentasterias, 
and the four-armed ones are possibly the same as Staurastrum. This, however, is only a 
conjecture, hazarded without having seen authentic specimens or good figures of those genera. 
The structure of the arms is exactly as in the Xanthidium, (Fig. 15, PI. 21,) and there is 
indeed an evident relation between the genera. 
The reader is requested to compare some of the figures last referred to, with the descrip¬ 
tions of Desmidium hexaceros and D. aculeatum. 
Euastrum -. (Fig. 14, PI. 21.) Binary, corpuscles triangular, each angle terminating in a sharp 
spine. Hab. West-Point. 
I have seen several other species of Euastrum, but the number figured is sufl^cient to give 
an idea of the variety and beauty of the forms in this interesting genus. 
Closterium. (See Figs. 30 to 38, Pi. 21.) Ehrenberg makes of this genus a distinct family of In¬ 
fusoria, which he calls the Closterina, and characterizes thus: Polygastric {distinctly or frohor 
bly) without alimentary canal^ without appendices; polypidoms having the form of a wand 
{‘^baguette"), thread or spindle^ by spontaneous division; papilla fixed and movable in the open¬ 
ing of the carapace. 
I have before stated that I consider the genus Closterium most closely related to Euastrum, 
and therefore with the Desmidiacea generally. This relation to Euastrum is manifest in their 
apparent identity in internal structure, the chief difference between them is only in the external 
forms ; and even in them, we find there is a perfect transition from the highly lobed and tabu¬ 
lar forms of some species of Euastrum, to the entire elongated and fusiform species of Clo¬ 
sterium. It is therefore without hesitation that I place Closterium, (as indeed most writers 
do,) among the Desmidiacea. 
There has been much discussion of the question, whether the Closteria are plants or ani¬ 
mals ; and as this inquiry is one of general interest, the decision of which will afiect the posi¬ 
tion of all the family Bacillaria, I may be excused for giving at some length an account of the 
present state of the question. Ehrenberg gives the following reasons for believing the Closte¬ 
ria to be animals: 1. Their voluntary motion; 2. Their terminal openings; 3. The inces¬ 
santly moving organs placed against the openings, and sometimes projecting; 4. Their sponta¬ 
neous lateral division. 
Morren, is his celebrated memoir “ Sur les Closteriees,” (some notice of which may be 
found in Silliman’s Journal, Vol. 35, p. 122,) supports the view that the Closteria are plants. 
