186 MEMORANDA. 
is formed of species of Plewrosigma in the same predicament. 
On the otber hand, the entire genus Cyméella is habitually 
reversed, although specimens may occur assuming accidentally 
the appearance of a Pleurosigma. 'These accidental appear- 
ances are sometimes caused by actual resupination, some- 
times from preparing to form spores, and sometimes by a 
mere change of position. 
Microscopical differences are by themselves of little im- 
portance. To see is one thing; to understand and combine 
what we see, another : the eye must be subservient to the mind. 
Every supposed new species requires to be separated from its 
allies, and then subjected toa series of careful observations 
and critical comparisons. To indicate many apparently new 
species is the work of an hour, to establish only one on a sure 
foundation is sometimes the labour of months or years. In 
microscopical natural history as much scrutiny is required to 
prove a new form to be distinct from its allies as in chemistry 
to discover a new alkaloid, or in astronomy to demonstrate 
the identity of two comets. A naturalist cannot be too 
cautious. It is better to allow diatoms to remain in the 
depths of the sea, or in their native pools, than, from imper- 
fect materials, to elevate them to the rank of distinct species, 
and encumber our catalogue with a load of new names so ill 
defined, if defined at all, that others are unable to recognise 
them; the same object can be more easily attamed by attach- 
ing them, in the mean time, to some already recorded species, 
with the specific character of which they sufficiently accord. 
In all such cases the question to be solved for the advantage 
of naturalists is not, whether the object noticed be a new 
species, but whether it has been proved such, and clearly 
characterised.—E. Watker-Arnort, Dowanhill, near Glas- 
gow. 
On the Structure of Rhabdonema and other Diatomaces 
with compound frustules.—It is with much reluctance I take 
up my pen to make the following remarks on this subject. 
During the progress of the ‘ Synopsis of British Diatomacee,’ 
by the late Professor W. Smith, a frequent correspondence was 
maintained between us, in which he constantly acknowledges 
the value of my assistance in his investigations, and promised 
to give me due credit for it in the work. 
The difficulties attendant on a clear comprehension of the 
structure of the frustule in the genera in question were con- 
siderable ; like nearly all the Diatomacez they are too minute 
for actual dissection, and whilst very opaque dry, the appear- 
ances presented in this state, so far as they could be made out, 
