WALKER-ARNOTT, ON ARACHNOIDISCUS. 163 
the siliceous one, and that by too long boiling in acid, as is 
necessary for guano, the marks are much obliterated, or en- 
tirely removed. This, however, is not peculiar to the present 
genus, but may be observed, more or less, in all diatoms, 
although sometimes the vegetable pellicle is very thin, and 
may be removed by a few seconds’ immersion in boiling nitric 
acid. Itis this circumstance which gives a quite different 
appearance to the same species, according as the preparafion 
is made. Thus, in Actinocyclus the vegetable epidermis is 
cellular, while the siliceous part is striated like a Pleurosigma ; 
and when the vegetable part is removed, we often find nodules 
or knobs along the margin (forming, then, the genus Ompha- 
lopelta), not previously visible. ‘Those who describe diatoms 
from slides are thus hable to commit great errors, and indeed 
no certainty can be obtained, except by getting the recent or 
growing diatom and examining it—lst, after being im- 
mersed for a short time im cold acid, or simply washed in 
boiling water; 2dly, after beimg boiled im acid for about half 
a minute, or a whole minute at most; and 3dly, after being 
boiled for a considerable time. We shall then see that many 
of the supposed distinct species of authors are the same, pre- 
pared in a different way. Of course deposits or guanos can 
yield little or no information; although once a species has 
been determined by the way I have indicated, we may be 
able to refer forms occurring in guano or deposits to it, with 
tolerable certainty. 
In my paper on Rhabdonema, in the last number of this 
Journal, I described the genus Eupleuria: since then I have 
found E. pulchella, not uncommon on Ballia, from Cape North- 
umberland, in South Australia. In that paper I noticed that 
E. incurvata differed from the others by the annuli not being 
cellular ; it is therefore probable, that it will have to be re- 
moved from the genus, particularly if the supposed annuli in 
that species prove to be only the siliceous connecting zone 
split, as it occasionally does in various other genera, into thin 
lamina. That this may be its true structure is rendered pos- 
sible by the discovery of a new genus from Mauritius, growing 
on Plocamium Telfairie (along with the Arachnoidiscus). This 
new genus has certainly no annuli: the upper and lower 
valves are as described in Hupleuria, and consequently it is 
intermediate between that genus and Achnanthes; differing 
from this last by the want of a stauros to the lower valve; 
by the costze not proceeding to the extremities, at least, on 
the lower valve; and by the valves being merely arched, 
and not geniculate; it has no stipes, and seems attached by 
the side, as in Kupleuria. To this genus the name Gephyria 
