20 On the Structure of Diatoms. 



microscopists have written upon this subject, that the writer would 

 fain be silent were it not for a firm belief in the superiority of the 

 instrument he used, for this kind of investigation. In fact this 

 excellent glass gives advanced work on almost every test tried, and 

 fully justifies the confidence reposed in it. The observations re- 

 corded below, unless where otherwise stated, were made with a 

 Tolles' :^V immersion objective of 165° angle of aperture, and gene- 

 rally a Tolles' 2-inch eye-piece, giving an amplification of 2500 

 diameters. 



Eupodiscus Argus. — My attention was especially called to this 

 shell by having noticed the wide difference between the views of 

 Mr. Henry J. Slack, Mr. Samuel Wells, and Mr. Charles Stodder. 

 My observations are corroborative of the idea of two plates, as 

 asserted by Messrs. Stodder and Wells. Using a | objective with 

 power of 340 times obtained by high eye-piece and extending draw- 

 tube, and using a Lieberkiihn, the outside or coarser markings on 

 specimens mounted convex side uppermost are white with white 

 cloud Olumination. An erased space on one shell and the holes or 

 depressions through which Slack's four large " spherules " are seen, 

 are now black. They, then, are not covered by the external 

 "crust." 



The slide was then turned over and the inside of the same 

 specimen examined by the same method, and on the more favour- 

 able portions of it the finer network of the inner jilate is also seen 

 in white, the " spherules " being perfectly black. By this reflected 

 light the " four spherules " are plainly seen to be dark openings in 

 the white plate, and the network is clearly traced across the areolae 

 in the outside plate. The diatom looks like a piece of coarse white 

 netting laid over a finer piece. 



Under the Tolles' -^-^ and with transmitted light, whether central 

 or obhque, it matters not, all portions of the surface of both the 

 upper and the lower plates are found to be covered with, or com- 

 posed of, a still finer network with irregular oval meshes hke the 

 two coarser ones. 



The place in the shell above referred to, where the layers are 

 erased, denuding an interior structureless " vail," gives an oppor- 

 tunity to observe the edges of the fractured layers. The broken 

 edges of both plates bordering on the erasure show the jagging of 

 this finest net structure. The arrangement of the finest areolae is 

 more regular near the margin of the diatom, or appears so by 

 reason of the simpler character of the structure in that part. They 

 are easily seen with the 5^0 on any part of every specimen studied, 

 but are unusually distinct between the " four spherules " on the inner 

 plate looking through the largest openings in the outer plate ; or 

 may be rendered still more distinct on shells with the concave side 

 up. They are more difl&cult to be seen on the outside crust with 



