surface-markings of diatom ace.e. 



281 



who have most carefully studied this question with, the most perfect 

 instruments, in the belief that the areolae are minute tubercular 

 elevations, the intervening network being formed by the thinner 

 portion of the valve.* This view, which is based on the optical in- 



Fig. 120. 



Outline of Pleurosigma quadratv.m, as seen under a power of 400 

 diameters : — at a, b, d, are shown the directions of the lines seen 

 under a power of 1,300, the illuminating rays falling obliquely 

 in each case) in a direction at right angles to the lines ; at e 

 are shown two sets of lines as seen when the oblique rays fall 

 in the direction of the midrib ; and at c is shown the appearance 

 of the markings when illuminated with an Achromatic Con- 

 denser of large angular aperture, the portion left blank showing 

 the obliteration of the markings by moisture. 



* See Dr. Wallich's Papers on this subject in " Quart. Journ. of 

 Microsc. Science," Vol. vi. (1858), p. 247 ; " Annals of Nat. Hist.," Vol. v. 

 Ser. 4 (Feb. 1860), p. 122 ; and "Trans, of Micr. Soc," Vol. viii., N. S. 

 (I860), p. 129. See also Norman in "Quart. Journ. of Microsc. Sci.," 

 Vol. ii., N. S. (1862), p. 212.— Mr. Wenbam, who at one time inclined to 

 the belief that the areolae are depressions, stated (when Dr. Wallich's 



