56 
GEOLOGY OF THE FIRST DISTRICT. 
Fossilen Infusorien und die Lebendige Dammerde,” PI. 21, figs. 2, 12 to 17, which show the 
closest resemblance between the recent and the fossil species. It is remarkable, however, 
that the recent species are inhabitants of fresh water, while flint is undoubtedly of marine 
origin. I have not seen distinct motion in any of our species. 
Xanthidium ? ramosum. Corpuscles globular, separate or binary, spines scattered, terminated in three 
or more points, -g’g to line. Fossil in gun flints. 
Xanthidium -. (Fig. 15, PI. 21.) Binary, each portion having numerous rather long arms 
terminating in three diverging points. 
This very beautiful American species is, when living, of a fine green color. Its carapace 
is hard and apparently siliceous, as it retains its form in spite of the action of fire and acids. 
Every living specimen of this species which I have seen, has been composed of two symme¬ 
trical portions, as shown in the figure. These bodies, when in the position shown in the 
figure, have much resemblance to Ehrenberg’s drawings illustrating the spontaneous division 
of some of the species. When thrown into other positions, so that the line of union of the 
two portions is not seen, it may easily be mistaken for a simple spherical body bristling with 
arms. The two portions often separate after death, and may then be mistaken for individuals 
resulting from spontaneous division ; but by throwing them into various positions by means 
of a compressor, the orifice corresponding to the line of separation of the two original parts 
may always be seen. The same remark will apply to the next species also. 
It occurs not unfrequently in a subalpine pond a few miles from West-Point, and at Salem, 
Mass.; and it is also occasionally found in ditches, in peat bogs, etc. 
Xanthidium -. (Fig. 16, a, b, PI. 21.) Binary; each portion somewhat triangular, gnd termi¬ 
nating at each angle in three short diverging arms, each having three small diverging points. 
The smaller size and triangular form of these bodies, make me think them of a different 
species from the preceding. I have not seen any intermediate forms, although they occur 
together in about equal abundance at the above mentioned locality. 
I must here remark that a transition from Xanthidium to Euastrum appears quite evident, 
through the binary triangular bodies, often having projecting arms, represented in figures 2, 
3, 4, 9 and 8, PI. 21. 
Arthrodesmus. Free, carapace simple, univalve, compressed in the form of a plate or a compressed 
band, articulated by spontaneous division. 
Arthrodesmus quadricaudatus. (Fig. 17, PI. 21.) Corpuscles oblong, straight; chain or polypidom 
of four to eight individuals formed by imperfect spontaneous division; four horns, (the middle cor¬ 
puscles rounded at the ends, the others with a horn at each end.) Size of corpuscles to 
line; chains yU line. Scenedesmus magnus, Meyen. 
This species, as it occurs in this country, is composed of from four to eight green elliptical 
corpuscles arranged in a thread or chain, usually having horns only on the two extreme cor¬ 
puscles, but not unfrequently similar projections may be seen on the middle ones also. Meyen, 
who considers this as a plant, states that in spring it contains starch globules.* It is exten 
Meyen, Pflanzen Physiologic, Vol. 3, p. 437. 
