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POPULAR SCIENCE REVIEW. 
the two “ median lines ” which consist of hemispheres as large as 
those in the outer row of the border. In the centre of the 
valve the bos or umbilicus pushes out the adjacent beads of the 
median rows into an oval form. Curiously enough, at the pre- 
ceding meeting of the Microscopical Society I had alluded to a 
supposed triumph of the improved kettledrum in resolving the 
rhomboides into e{ dots as black as jet,” which, though admired 
by Dr. Millar and myself, are now proved, under prism illumi- 
nation, to be an unnatural arrangement of light and shade. 
PLEUROSIGMA FORMOSUM AND PLEUROSIGMA STRIGILIS. 
In Pleurosigma formosum the rows of silicious hemispheres 
are at right angles to each other and meet the longitudinal 
division of the valve at an angle of 45°. In one direction there 
are 24 hemispheres and intervals in the 12-inch diameter of the 
field, i.e. in ± oVoth of an inch, and in the direction at right 
angles to it there are 30 hemispheres and intervals, so that the 
rows of equal hemispheres are rather closer together in one 
direction than in the other. Here under the magnifying power 
of 12,000 linear one hemisphere and interval occupy half an 
inch, the apparent diameter of the hemisphere being y^ths 
and of the interval T 2 yths of an inch ; this, of course, makes 
the real diameter of the hemisphere y ^ Q- t h of an inch, and there- 
fore, with P. strigilis , among the largest in the range of val- 
vular structure. We are told popularly that the “ strice ” of 
diatoms range between about 30 and 100 in y-^oth of an inch, 
and, to adduce an example, that the striae of P. strigilis are 
much closer than those of P. formosum — a statement which 
gives no idea of the fact that the diameters of their hemispheres 
are the same. Doubtless it would be more accurate to substi- 
tute the terms hemispheres and intervals for <c striae and lines,” 
and to give the number of hemispheres in y-^ -^th of an inch 
and the measure of the space between them, or the ratio of the 
diameter to the interval. So well does P. formosum tell its 
own tale and set at rest the question of elevations or depres- 
sions, that a young friend of mine, under fourteen years of 
age, exclaimed, when he saw it, that 66 it looked like a plate of 
marbles.” 
PINNULARIA MAJOR AND PLEUROSIGMA EALTICUM. 
In Pinnularia major the “ stout costae ” follow the law of 
structure and consist of very closely-packed spheres, and in 
Pleurosigma balticum the markings, regarded by Dr. Wallich 
as representing four-sided flattened pyramids, follow the same 
type, only a larger portion of the sphere crops out above the 
surface of the valve. 
