Diatoms. 335 
ereatest variety. Bermuda earth contains one most beau- 
tiful object, the Heliopelta (sun-shield), of which a tolerable 
notion may be got by cutting an orange in half transversely. 
Then every alternate triangle you must suppose to be 
marked with a different pattern—one being covered with 
large and regular round markings; and the next, which ap- 
pears to be on a different level, to be marked with smaller 
and less distinct, but nevertheless, very beautiful markings. 
The ribs which divide the triangles from one another, dilate 
at their extremities, forming in the centre a clear space 
corresponding to the central pith in the orange, and at the 
ends next the margin expand and gradually melt into the 
rim or border, which is thickly set round with transparent 
spikes of different lengths. The earth from Richmond 
affords many beautiful specimens, especially of the genus 
Navicula. 
Guano, the dry excrement of sea-fowls, is very rich in 
objects. One, the Avachnotdiscus, is like a small and _ per- 
fect spider's web (whence its name), with all the colours of 
the rainbow condensed init. A large variety, or indeed 
two or three varieties, of the Coscinodiscus are present in 
considerable numbers; and the Zygoceros rhombus is a 
miniature shepherd’s purse, such as is found on the sea- 
shore, only covered with dots. The Actenocyclus is the 
same kind of thing as the He/zopelta, only without the mar- 
ginal spines. The earth of Oran contains the same Dia- 
toms as guano. The guano* and the earths mentioned 
are to be prepared in the same way as the sediment from 
the weeds from Southend, only they should be well washed 
in water first (the guano more especially). The modus 
operandi is to shake up the earth or guano with water inan 
oil flask, and then allow it to settle: this is to be repeated 
until the water is no longer coloured. It is then to be 
treated as before directed. Most writers recommend the 
use of hydrochloric acid first, and secondly, nitric acid, 
when the former ceases to act. With large quantities this 
_ would be more economical ; but it necessitates the purchase 
of a second stoppered bottle. 
The reader exclaims, “Well! Now I have found these 
things, what are they?” The writer answers, ¢hatz is a sub- 
ject of dispute, some claiming them for the animal world 
and others for the vegetable. The chief argument for their 
belonging to the animal kingdom, is their voluntary motion ; 
* The guano can be had for 4d. the lb. at Butler’s, in Covent Garden. 
