THE 
MONTHLY MICROSCOPICAL JOURNAL. 
SEPTEMBER 1, 1870. 
I.—On the Structure of the Pleurosigma angulatum and Pleuro- 
sigma quadratum. By Joun Antuony, M.D., Cantab. 
(Read before the RoyaL Microscoricau Society, June 8, 1870.) 
Prats LIX. 
I wave to call attention to a peculiarity in the structure of the 
P. angulatum and its allied species, which, so far as I know, has 
never been described. Taking the fact that few microscopic objects 
have been so frequently and so critically examined as the silicious 
valves of these beautiful diatoms, not only for the purpose of 
determining the precise nature of the markings which shaseetene 
the Loricz themselves, but by way of testing the appliances of the 
microscope, it might fairly be expected that we had fully made out 
and put on record all that was worthy of note with respect to these 
shells, but that such is by no means the case I trust to be able 
to show clearly in the course of this paper. I will begin by saying, 
that I feel grateful to these minute objects for the lessons they have 
taught me in the use of the microscope; more particularly in the 
management of light so as to distinguish between the true and the 
false ; and that I have studied these silicious scales for years, from 
the far-away time when I was proud of being able to make out 
crossed lines on their surface with the aid of one of the earliest ;1,ths 
constructed, until now, when I am even more pleased to be able to 
show the nodules, or “ dots,” as we used to call them, most brilliantly 
by means of a modern 1th. Having then studied these objects so 
long and so constantly, I trust it will not be thought that the 
observations I am about to make are either hasty or given without 
due consideration. The Pleurosigmata have usually been divided 
into two sections, v2z. into those in which the striz or rows of dots are 
arranged “ obliquely ” with respect to the axis of the shell, and into 
those which are arranged “ transversely and longitudinally,” or as I 
should prefer to call it “rectangularly.” In the first or oblique 
division, the Pleurosigma angulatum and P. quadratum have been 
unhesitatingly placed, while the second or rectangular division has 
received P. hippocampus, P. Balticwm, and others well known to 
VOL. IV. K 
