INFUSOEIA. 57 
sively diffused, both in Europe and in this country: I have noticed it as far west as Ouiscon 
sin, and south to Virginia. It is abundant in ponds near West-Point, N. Y. 
Arthrodesmus culm. (Fig. 18, a, PI. 21.) Corpuscles green, oblong, alternating in a right line, by 
spontaneous division. 
The small green bodies represented in our Fig. 18 , occur in the well at Fort Putnam, West- 
Point. They appear to belong to this species. 
Odontella. Free, carapace simple, univalve, compressed, form of flattened articulated ribbands, 
often pierced, produced by imperfect spontaneous division, articulations united by small projec¬ 
tions. 
I have not satisfactorily determined any American species of this genus, although I have 
occasionally seen small filaments which agreed tolerably well with the generic character. 
Micrasterias. Free, carapace simple, univalve, compressed; groups of a number of individuals 
in form of a flattened star, produced by imperfect spontaneous division. 
This is a very beautiful genus, its species presenting elegant star-like arrangements of 
green corpuscles, some of which closely resemble in form the stars and badges of honor worn 
in Europe. 
There appears to me to be much confusion in the specific characters, arising from the 
circumstance that the number of corpuscles in the different rows has been made a character 
of specific importance. From what I have seen of the species, I am satisfied that the num¬ 
ber of corpuscles in a star is liable to great variation in the same species. Perhaps the form 
of the corpuscles would prove a more certain character. 
Micrasterias tetras. (Fig. 19, PI. 21.) Four corpuscles united in form of star, the edge slightly 
notched. 
This is a very minute species, which occurs in ponds near West-Point. I have also noticed 
it in Virginia. 
Micrasterias boryana. (Fig. 20, PI. 21.) Ten corpuscles in the exterior circle, five in the interior, 
and one in the centre, edge acutely dentate. Corpuscles Jp to of line. 
It happens accidentally that our figure represents an individual having just the number of 
corpuscles above described. I am satisfied, however, from frequent observation, that the 
same species sometimes has a much greater number of corpuscles, certainly another row of 
fifteen is often developed. It then agrees with M. tricyclia of Ehrenberg. It is a very beau¬ 
tiful object for the microscope. I have found it in New-York, Virginia, and in Ouisconsin. 
Micrasterias -. (Fig. 21, Pi. 21.) Corpuscles very numerous, forming large imperforate plates 
of a circular or elliptical form; exterior corpuscles deeply emarginate, each having two projecting 
points. 
This large and very beautiful species is not uncommon in ponds near West-Point. 
Eitastrum. Free, carapace simple, univalve, compressed, binary, sometimes quaternary, having the 
form of a two-lobed disc or lamina, often dentate. 
Geol. 1st Dist. 8 
