Dr. Schumann s Formulae for Diatom-lines. By W. J. Hiclcie. 15 
If we compare the special case of 1 with this, we find there 
e = a J 2, and here c = ^ ^ 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. sphserophora, 
Wien, 1860, II, 84 ; Gocconeis clecussata, Elirenb. Amer. II. vi, 
13 ; Cocc. placentui a and oceanica. 
Many species of Pleurosignna at least approximate to this case, 
as the inclination of their oblique systems of lines to the middle 
line does not amount to 60 J , but sinks towards 45°. 
As a case in point, I mention Fleur, elonyatum. 
One specimen of Stauroneis nobilis m. gave 
a = 30, 
b = c = 21 ; 
Another gave 
« = 27, 
b = c = 19; 
A third gave 
« = 3H, 
HC4 
CM 
II 
II 
-C> 
As 21.V2= 29-7, and 19.^2 = 26-9, and 22£ V2 = 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, 
o 
