STRUCTURE OF C0SC1N0DISCUS AsTEROMPHALUS, ETC. 157 



Mr. Smith's second paper (October 1912) I cannot touch : the 

 exigencies of space forbid. 



I am indebted to two members of the Club for the loan of two 

 slides realgar mounts which have enabled me to study the 

 minute structure of Pleurosigma angulatum and Pleurosigma 

 balticiua. Having taken several photographs from a slide lent 

 me by my friend Mr. Bruce Capell, I showed some of them to 

 our Secretary and Editor, and the latter informed me that 

 Mr. Nelson was engaged more or less on the same subject, and 

 suggested that I should send him copies of my photographs. 

 This suggestion I fell in with the more readily inasmuch as it 

 would give me the benefit of any adverse criticism which Mr. 

 Nelson might feel himself called upon to make. To elicit this I 

 wrote on the back of each a brief interpretation of the structure, 

 and in one case in which I was much puzzled I placed a note of 

 interrogation. With his usual kindness and urbanity, Mr. Nelson 

 gave the desired information, but instead of adverse criticism he 

 sent me two slides of great historic as well as intrinsic value. 

 From these I have been enabled to make some photographs 

 which confirm the results already obtained from the slide belong- 

 ing to Mr. Bruce Capell. 



Coming now to my immediate subject, my readers are, no 

 doubt, aware that Dr. Van Heurck tells us that a diatom valve 

 consists of two membranes and of an intermediate laver which he 

 calls a septum, and that it is this latter layer which contains the 

 cavities or perforations. In my opinion this definition connotes 

 at once too much and too little : too much by giving the valve 

 three layers, and too little by confining the cavities to the septum 

 only. In the three valves which we are about to consider, I find 

 only two layers or membranes, each of which has its own perfora- 

 tions. We will, in the first place, consider the structure of 

 C oscinodiscus asteromphalus. The photographs are taken from 

 some of my own mounts in styrax. Of course I am aware that 

 this valve was very ably and fully treated recently elsewhere by 

 Dr. Butcher (Journ. R. M. S. 1911, p. 722), but my chief object 

 in bringing it before you now is to determine, if we can, which is 

 the correct image, the black dot or the white dot. [Here Mr. 

 O'Donohoe illustrated his remarks by photographs projected on 

 the screen.] The black- and white-dot images now thrown on the 

 screen have been taken direct at a magnification of 4,000 

 diameters, and to my mind it seems perfectly obvious that two 

 images so utterly unlike one another cannot both be correct 



