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Transactions of the 
III . — On the Structure of the Valves of Eupodiscus Argus and 
Istlimia enervis, shoiving that their silicious deposit conforms 
to the general plan of deposition in simpler forms. By 
Henry J. Slack, F.G-.S., Sec. R.M.S. 
(Read before the Royal Microscopical Society, Nov. 6, 1872.) 
Plate XL. (Middle portion). 
The observations now brought before the Royal Microscopical 
Society may be regarded as a continuation of those previously 
communicated on Pinnulariae, as they have the same object, and 
tend towards the same conclusion that the silicious deposition in 
diatoms probably follows one uniform plan, and that the silex is 
deposited in spherules, varying in dimensions and in degree of 
proximity ; thus according with Mr. Wenham’s researches. 
Since the publication of the paper on Pinnulariae numerous fresh 
observations have been made, confirming the results then mentioned, 
and efforts were occasionally directed towards elucidating the 
structure of such forms as Istlimia enervis and Eupodiscus Argus, 
and showing their agreement with the ordinary types ; but without 
satisfactory results so long as only old means were employed. 
Lately, with the help of Mr. Wenham’s “ Improved Reflex 
Illuminator,” described in a paper read before this Society last May, 
and published in the June number of this Journal, the markings of 
Eupodiscus Argus have been so clearly displayed as to leave little 
doubt of the real structure, and those of Istlimia enervis have been 
exhibited so as to indicate that the aberrant character shown under 
ordinary illumination is the result of a peculiar arrangement of 
minute beads viewed under circumstances of optical confusion. 
No thoughtful microscopist could be satisfied with the views 
Eupodiscus Argus obtained by ordinary means, and corresponding 
in general effect with the figure given in the ‘ Micrographic Dic- 
tionary,’ pi. 12. In Smith’s work on the Diatomaceae a better 
figure was given, but far from being right, though at the time of 
its publication there was probably no particular reason known for 
doubting its correctness. 
The figures in the ‘ Micrographic Dictionary ’ (2nd edition) 
would suggest to anyone familiar with the confusion arising from 
defective illumination and the superposition of transparent sur- 
faces that it must be wrong. It represents a number of bright 
objects something like white stars with short rays, on a dark 
ground of general confusion. In the text of the same work the 
genus Eupodiscus is described as having valves that “ appear either 
distinctly areolar, the depressions being large : granular from their 
