266 THE AMERICAN MONTHLY [Oct 
visible at 775 diameters, and the lines across the middle 
are merely a very coarse imitation of them, to show their 
direction, ete. 
-Mr. Patrick informed me that he very carefully ex- 
amined the alga the Amphipleura was growing on and 
found it to be Cladaphora fracta Kg., which I believe 
only grew over a small space on the east and north walls, 
a fact very interesting, as showing that it is probably 
parasitic on this alga, and only found in connection with - 
it, something not before observed, so far as known to me. |. . 
This would account for its not being found under the 
lily-pads, or at the south end, where this alga did not 
grow. The rarity of this diatom may be due to the fact: 
that this alga does not grow every where. | 
I once measured Amphipleura pellucida by a Rogers 
stage micrometer, and found it not quite one two-hun- 
dredths of an inch in length. The smallest grains of 
ordinary sand which can be picked up with a pair of 
watchmaker’s tweezers and arranged as close together as 
possible under a magnifying glass go only sixty-four to 
an inch, so that the length of this diatom isonly a little 
over one-quarter of the diameter of one of the finest grains 
of sand; yet in this short length it is marked with 340 
of the finest and most regular lines ever seen ruled across 
it, and each line apparently composed of rows of beads. 
I counted these lines on an excellent photograph of it, by 
Doctor Detmers. A list of the genera and species found 
at Gage’s pond is as follows: 
- Achnanthes minutissima, Amphipleura pellucida. Amphora libyca; 
ovalis. Cocconema australicum; cistula; lanceolatum; mexicanum ; 
a large unknown, perhaps new. Cymatopleura elliptica; solea (both 
long and short). Cymbella gastroides; stomatophora; turgidula. 
Denticula elegans; tenuis; thermalis. Diatoma tenue. Encyonema 
lunula;turgidum. Epithemia gibba; gibba, var. ventricosum ; sorex, 
short form. Eunotia gracilis; lunaris; lunula. Fragellaria inter- 
media ; mutabilis. 
Gomphonema abbreviatum ; affine ; affinis ; angustatum ; angustatum, 
