ZOOLOGY A.ND BOTANY, MICROSCOPY, ETC. 



839 



the focus being altered slightly to take the latter. (In the figure 

 the outer edge of the cast is too hard and defined.) They are 

 represented by Fig. 148. The differences between siliceous frustule 

 and cast are : — In the former, every portion is sharply and hardly 

 defined. There is no appearance of beading ; altering the focus 



Fir,. 148. 



Fig. 149. 



brings out shading, and different planes of definition, showing 

 convexity and different curves. In the latter, the median line is 

 very irregular and interrupted, though clearly defined. The striae 

 are thicker, not hard edged, and show signs of beading. In many 

 places they coalesce. The whole is well defined at the one focus, 

 proving it to be comparatively in one plane. 



In the beginning of this year, when examining for selection a 

 mixed strewed (dirty) slide from a gathering, I found a Navicula 

 (Didyma ?), of which I have sent three photographs. It is re- 

 presented in Fig. 149. Bound it there appears to be a double 

 gelatinous membrane, with radial arms extending from the siliceous 

 frustule to the margin of the membrane, where they appear 

 thickened. At A the arms of the lower membrane seem to show 

 through the upper membrane, and form V's with the arms of the 

 latter. They are finer than those arms which appear single, but 

 which I conclude are formed by the arms of one membrane covering 

 those of the other. At B both membranes appear, the edges forming 

 loops. At C the membrane and arms are much extended, the arms 



