155 



THE MINUTE STRUCTURE OF COSCINODISCUS ASTER. 

 OMPHALUS AND OF THE TWO SPECIES OF 

 PLEUROSIGMA, P. ANGULATUM AND P. BALTICUM 



By T. A. O'Donohoe. 



{Read May 28th, 1913.) 



Plate 14. 



In preparing this paper it was, at first, my intention to refer in 

 no way to the work of others, of which, in fact, I had very little 

 knowledge. It has, however, been pointed out to me that it is 

 desirable to mention previous researches, in order to enable the 

 reader to compare these more easily with my own. Happily 

 Mr. E. M. Nelson gives a brief summary of his work on the 

 valve of Pleurosigma in a note read at the Club on January 

 28th, 1913 (Journ. Q. M. C, vol xii., p. 98). This note is 

 therefore easily accessible to all my readers, and any further 

 reference to it by me would be unnecessary. I regret I cannot 

 so easily dispose of the observations of Mr. T. F. Smith, who 

 has for many years devoted much time and thought to the 

 structure of diatoms, and who has so recently as August 1911 

 and October 1912 contributed to Knowledge two papers on this 

 much-discussed subject, entitled " The True Structure of the 

 Diatom Valve." These papers contain very many photo- 

 micrographs, of which several are excellent. I very much 

 regret, however, that I cannot agree with what I must call 

 his heterodox views. In his own words, "The points desired to 

 be driven home in the present article are that diatom structure, 

 consists of neither beads nor perforations as commonly under- 

 stood " (page 291). 



Speaking of Pleurosigma for mosum, he says: " It appears to 

 consist of a series of chains, as it w-ere, formed of short bars or 

 fibrils of silex, arranged lengthways on the valve. They run in 

 pairs, parallel, each pair having larger and narrower interspaces 

 between them in regular succession, and so placed that the larger 

 interspaces are set obliquely to the corresponding interspaces 

 between the other pairs both above and below." A few lines 

 farther on he tells us that " his theory is this, that what we see 

 in the Pleurosigma valve when sound is not the structure at 

 all, but simply a collection of focal images thrown from the other 



