46 
Transactions of the Royal Microscopical Society. 
so. It is only from the examination of authentic specimens that it 
is possible to ascertain the form to which Smith gave the specific 
name of curvula. Gregory gives a good figure, but no description. 
“ Dr. Lewis, U.S.A., in his paper on ‘ Some new and singular 
intermediate forms of Diatomaceae,’ has properly removed this form 
from the genus Nitzschia and placed it with the Surirellae. He 
gives the following specific characters : 
“ Surirella intermedia, n. sp. Frustules free. Yalve linear, 
strongly sigmoid, with attenuated rounded apices. F. Y. straight 
or slightly sigmoid, expanding at the sub-truncate extremities. 
Alas usually distinct, twisted near the ends of the valves, giving rise 
to a spathulate appearance. Canaliculi numerous, inconspicuous, 
reaching the narrow central blank line. Striae distinct, variable as 
to number and fineness.” 
The value of the distance of the striae on the Diatomaceae either 
as a test for the resolving powers of objectives or as specific distinc- 
tions is but little ; in the latter they may he said to be valueless, and 
in the former case they are only of value when the actual distance 
of the striae is known. Moller’s slide of N. curvula (= N. sigma) 
is mounted from a brackish water gathering from Schleswig and con- 
tains the same forms as my own Felixstow gathering, viz. N. sigma 
very plentiful, Pleurosigma fasciola common, P. angulatum, 
N. minutula, &c. ; hut in my gathering, the striae on the two first- 
named forms are resolvable with a £th objective of 75° angular 
aperture made twenty-two years ago by A. Eoss. On P. fasciola I 
can resolve both sets of striae, and with Beneche’s No. 7 and 
0 ocular I can see both sets at once (that is to say, the dots are 
shown in squares). The striae on the same forms on Moller’s slide 
1 can only resolve with my |th, N. sigma with difficulty, and only 
one set of striae at a time on P. fasciola. 
In Moller’s beautiful preparation of P. angulatum the striae on 
that form are easily resolved with my Eoss Jth, whilst those from 
many British localities are quite invisible under the same conditions. 
Nitzschia sigmoidea (type) is not an easy test for a good |th, hut 
its var. A is easily resolved by a good 4 inch. 
This variation in the distance of the striae on the same species of 
diatom probably arises from a more vigorous growth in the frustule, 
and it will be generally found that the more robust the frustule is, the 
more distinct are the striae. An unscrupulous dealer in objectives 
has only to take advantage of this fact, and he can palm off inferior 
objectives by guaranteeing their being able to resolve certain so- 
called tests ; for example, a very inferior £th would show the striae 
on N. sigmoidea /3, or an |th of moderate quality would probably 
show the markings on the coarsely striated P. fasciola, which would 
be perfectly incapable of resolving those on Moller’s slide. 
