, I'll Transactions of the Society. 



point, for the washing-out process could hardly have been accom- 

 plished without leaving some trace of crenation or irregularity on 

 the inner rim of one of the many ringed openings of Morland that 

 I had photographed, both in the complete and fractured state. This 

 is Mr. Firth's reply : — 



" As to my specimens being over-cleaned, I can. only say that 

 anyone with this opinion can gladly have a slide for examination. 

 Over-cleaned specimens cannot be handled nor mounted properly, 

 so you can contradict this. I have long experience in cleaning 

 diatoms, and am always striving to improve my slides, and I can 

 say that ray present slides are better than those of ten or fifteen 

 years ago. The cement used for fastening the diatoms to the 

 cover-glass is different and better, and the styrax of much better 

 quality. I have requests for slides from Fellows of the Royal 

 Microscopical Society, but none have any special reference to Cos- 

 cinodiscus astcromphalus, though all have been well pleased with 

 those I have sent — generally, diatoms of beauty in marking, etc. — 

 and I feel that they appreciate my work, and would not agree that 

 they are in any way deficient, either through over-cleaning or im- 

 perfect mounting." 



Later I asked Mr. Firth to prepare additional mounts of speci- 

 mens taken from different portions of the Maryland deposit, and 

 in particular to provide me with some of fragments only, mounted 

 as flat as possible in different media of high-refracting index. At 

 various stages of this study I have had the advantage of the 

 counsel and advice of Mr. E. Leonard, who, after examining the 

 slides with the lenses and oculars I was using, was of the opinion 

 that they were first-class mounts of typical examples of Coscino- 

 discus asteromphalus. Most of these slides, prepared by Mr. Firth, 

 are on the stands exhibited here to-night, and as far as possible I 

 have tried to show you the diatom and the conditions under which 

 I was working. One point, however, militating against an abso- 

 lutely perfect rendering of the tertiary markings, is that, in some 

 cases, oil -immersion condensers have not been obtainable, thus 

 preventing the full display of the value and beauty of this 

 particular diatom. The clearest results have been obtained from 

 two slides mounted in a medium of monobromide of naphthaline 

 and styrax, these being closely followed by two mounted in 

 styrax and one excellent slide in liquidambar styraciflua. 



EXPLANATION OF PLATE XXVII. 



Fig. 1. — Fragment of the polygonal cell-layer, with the " ringed " openings of 

 Morland. Taken at six consecutive foci, x 1000. 



