DOTTED STRUCTURE IN PINNULARIAE. 109 



to or forming a part of the girdle, and other dotted membranes 

 attached to the costae, but I have no direct evidence of this. 



5. That the dots are concavities, as one might expect by 

 analogy, because fracture takes place across them, and when 

 seen sideways on the rounded edge of a curved membrane the 

 curve of the concave outline is obvious. 



The photographs were all taken with a 2 mm. Holos. objective, 

 N.A. 1'36 ; X 8 compensating eyepiece ; a Holos. immersion 

 condenser diaphragmed down to N.A. TO, except for oblique 

 light, then at full aperture of N.A. 1'38 ; and with green light. 

 The specimens are mounted in "realgar." 



Fig. 1. Pinnularia nobilis, x 1,300. Illuminated with a solid, 

 central cone N.A. 1*0. This shows dots through the costae and 

 along the whole length of each costa as far as the focus allows. 

 There is also evidence of out-of-focus structure between the 

 costae. 



Fig. 2. P. nobilis, x 1,200. A much smaller valve than above, 

 and one that shows no trace of structure with central light. A is 

 focused for a slightly lower plane than B. Doubly oblique light. 

 The dots are clearly visible along the whole length of the costae, 

 as the focus permits, and also between them. The dots are 

 veiled, I suggest by the membrane that lies over the costae, 

 and to a certain extent confused by the material that they have 

 to be seen through. 



Fig. 3. P. viridis, x 1,280. Illuminated with a solid central 

 cone, N.A. TO. The upper membrane here is missing in parts, 

 and where it is missing the dots show through the costae and also 

 between them. 



Fig. 4. Part of a valve with its inner or concave surface upper- 

 most, X 1,250. Illuminated with a solid central cone, N.A. TO. 

 Portions of dotted membrane are adhering to the upper (really 

 the inner as regards the frustule) edges of the costae. 



Fig. 5. These appear to be detached costae showing their 

 side faces, x 1,250. Illuminated with a solid central cone, 

 N.A. I'O. They show no sign of dotted structure. 



Fig. 6. A detached strip of dotted structure, x 1,200. Illumin- 

 ated with a solid central cone, N.A. TO. The whole length of 

 the strip, only part of which is shown, agrees with the length of 



