MEMORANDA. 261 
section. Let now a piece of thin paper be laid over the 
cover, and upon this a thick slide; if a moderate heat be 
applied to both the slides, over and beneath the specimen, 
direct pressure, evenly exerted with the finger (or spring 
clothes-pins), will force out all unnecessary balsam, and leave 
the section and the protecting cover perfectly flat and 
unbroken. 
The reader will not deem me too prolix when he attempts 
his first preparation, or when, after having followed the plans 
so scantily given in the books, he feels the need of something 
precisely definite. It is certam that neither Canada balsam 
nor gum mastic will retain the first ground side of a specimen 
upon a slide long enough to enable the preparer to reduce it 
to the requisite thinness, and with both these substances 
heat must be employed, which is objectionable, because most 
objects are thereby warped or cracked ; and, furthermore, the 
paper guard, which I hold to be indispensable for limiting 
and equalizing the thinness of a section, is not mentioned in 
treatises, in which, if known to the author, such a measure 
should be noticed. But it is possible to fasten agate, fossil 
wood, &c., with hot gum shellac, so that they may be ground 
upon both sides with a water stone; but even in these 
instances invidious cracks may endanger or destroy the 
beauty of a choice preparation. 
I am confident that my specimens are second to none in 
any respect; and the highly creditable performances of 
friends, to whom I have given the method, forming the sub- 
ject of this communication, lead me to believe, that with the 
facilities it affords, the observers of our country will need no 
Topping for objects within their reach, and I beg leave to 
add that the profitable pleasure I have enjoyed induces me, 
through the ‘ American Journal of Science,’ to invite parti- 
cipation.—CuristorHeR Jounston, M.D. 
On Actinocyclus and Eupodiscus.— Mr. Edwards, in a paper 
“On American Diatomacee,”’ read on the 3d March, states 
that he considers the Actinocyclus triradiatus, described by 
me in the ‘ Micr. Journ.,’ vol. vi, p. 23, to belong to the genus 
Coscinodiscus. I have, however, good evidence that this is 
not the case; and that, in fact, it is merely an internal plate, 
or perhaps an imperfectly formed valve, of the common 
Actinocylus radiaius of Smith. In some gatherings kindly 
forwarded to me by Colonel Baddeley, this species is very 
abundant, and the peculiar valves, which I had found pre- 
viously always in a detached state, occur on many of the 
perfect frustules, and I imagine are the newly formed valves, 
