58 
GEOLOGY OF THE FIRST DISTRICT. 
The elegant forms and emerald green color of the species of this genus render them exceed¬ 
ingly fine objects for the microscope. The forms, as usually seen, appear tabular; but when 
t hrown on their sides by means of a compressor, they show considerable thickness. 
I have noticed in several species, groups of molecules moving actively, precisely like 
those seen in Closterium. Indeed this genus is most closely allied to Closterium, and some 
forms occur which show a complete transition from one genus to the other. (See remarks 
under the head Closterium.) 
Capt. Carmichael, with his usual acuteness, detected their animal nature. He remarks 
of two of the species, “ these are animals instead of plants, if the faculty of locomotion will 
entitle them to that rank.” (See Hooker’s British Flora, Vol. 5, p. 398.) I have frequently 
noticed the motion of several species ; it is quite as distinct as in Closterium. 
Euastrum rota, Ehr. (Fig. 22. PI. 21.) Body binary, lenticular, discoid, smooth, the edges dentate 
or spiny, to line. Echinella rotata 1 Greville. 
The species represented in our figure appears to be the E. rota of Ehrenberg, and agrees 
pretty well with the account given by Greville of his Echinella rotata, which he describes 
as having the “ frond plane, circular, divided by a line passing through the centre, each 
portion composed of radiating segments cleft nearly to the central line.” (See Brit. Flora, p. 
398.) Having seen no figures of the European species, I cannot be sure of their identity 
with ours. 
The species represented in Fig. 22, is quite common in the United States. I have seen it 
in Rhode-Island, New-York, Virginia, and Ouisconsin; I have generally found it scattered 
among conferva;, but I once in early spring found many hundreds of them collected together 
on the bottom of a very small pool of water in a sphagnous bog. ‘ Some variety occurs in 
the outline ; thus the two large central portions of each half are often perfectly symmetrical, 
and not unfrequently dentate near the ends. I have seen specimens twice the size of the 
one represented. 
Euastrum crux-meliiensis. (Fig. 23, PI. 21.) Body binary, lenticular, discoid, smooth, the edges 
deeply divided into six dentate and spiny rays. 
I copy this description, that it may be compared with Fig. 23, PI. 21, which represents a 
very beautiful form which I have found in various situations near West-Point, and also at 
Staten Island. I have seen it move distinctly. 
Euastrum -. (Fig. 24, PI. 21.) This is possibly only a younger state of E. rota, (Fig. 22,) 
with which it occurs. 
Euastrum -. (Fig. 25, PI. 1.) This very elegant form is somewhat rare. It occurs at West- 
Point with the preceding. 
Euastrum -. (Fig. 26 and Fig. 27, a, b, c and d, PI. 21.) I suspect that the species represented 
in Figs. 26 and 27, is the same as the Echinella oblonga of Greville, which he describes as being 
“ compressed, oblong crenato-pinnatifid, and lobed, divided transversely almost to the centre.” (Brit. 
Flora, Vol. 5. p. 398.) 
Fig. 27, a and h, shows two positions of the same individual ; c and d shows small indi¬ 
viduals, which are probably the young of this species. Fig. 26, although much more deeply 
