Dr. Schumann s For mulse for Biatom-lines. By W.J.Riclde. 15 
If we compare the special case of 1 witli this, we find there 
c = and here c = 2 2, a ratio which furnishes us with an 
excellent means of distinguishing the structure of different diatom 
shells. 
To this class belong Navicula tumens and Nav. sphmrojohora, 
Wien, 1860, II, 84 ; Cocconeis decussata, Ehrenb. Amer. II. vi, 
13 ; Cocc. placenfuJa and oceanica. 
Many species of Pleurosigma at least approximate to this case, 
as the inclination of their oblique systems of lines to the middle 
line does not amount to 60'^, but sinks towards 45°. 
As a case in point, I mention Pleur. elongatum. 
One specimen of Stauroneis nohilis m. gave 
a = 30, 6 = c = 21 ; 
Another gave a = 27, 6 = c = 19 ; 
A third gave a = 31^ 6 = c = 21J. 
As 21.^2=29-7, and 19.^2 = 26-9, and 221^2 = 31-8, 
the measurements agree right well. 
Let this much suffice as a specimen of Schumann's modus 
operandi. Those who are desirous of a further acquaintance are 
referred to the original work. 
VOL. XIV. 
C 
