^SLIaSmSI Marhings on the Pleurosigma angulatum. 243 
XII. — On the Markings on the Pleurosigma angulatum and on the 
Lejpisma saccharina. By J. B. Dancer, F.E.A.S.* 
Whilst engaged in testing an object-glass on some injured valves 
of the Pleurosigma angulatum, 1 noticed a set of faint transverse 
lines on those portions of a valve from which the ordinary diagonal 
markings had been removed by abrasion. These transverse lines 
occupied the same positions on the valves as the ordinary ones, 
which are visible in perfect valves when oblique illumination is 
thrown in the direction of the length of the valve. The covering- 
glass of the slide had been cracked by rough usage, causing the 
partial destruction of the diatoms. At first it appeared probable 
that these were spurious lines, caused by moisture, which might 
have gained access to the valves through the crack in the covering- 
glass (such as had been noticed by Mr. Hunt, of Birmingham, and 
described by him in the * Quarterly Journal of Microscopical 
Science,' vol. iii.). The sHde was carefully dried by heat, and the 
lines were as visible as before. Eepeated observations appeared to 
prove that these faint lines really exist on the valves. There is 
little doubt but that the ordinary diagonal markings are on the 
outside, or convex surface of the diatoms, and in the specimens 
under examination the broken-up portions of the markings were 
scattered over the surface of the object. To account for the perfect 
preservation of the faint lines, I would suggest that they are on the 
under, or concave, surface of the valve. The lines are visible by 
oblique illumination, or with the achromatic condenser and central 
stop. Microscopists are sometimes forgetful that thin and dia- 
phanous objects have two sides ; I am reminded of a case in point : 
some thirty years back the scales of the Lejnsma saccharina were 
favourite test objects. The scales are very various in form, accord- 
ing to the position they occupy on the body of the insect ; some of 
the scales have a series of parallel lines running the whole length, 
which are frequently crossed by lines radiating from the point 
where the scale is attached to the insect. At the period named I 
was in the habit of frequently mounting Lepisma scales as objects 
for the microscope ; if too much pressure was applied in transfer- 
ring the scales from the body of the insect to the glass shde, one 
set of these lines was rubbed off. It thus became clear that the 
parallel lines were on one surface and the radiating ones on the 
other. About the year 1844 I exhibited the scales of the Lepisma 
in this condition, and gave this explanation to the late Sir David 
Brewster, who was on a visit to this town. Since then the late 
* Paper read before tlie Microscopical Section of tlic Manchester Literary and 
Philosophical Society, March 1st. 
