198 STRUCTURE OF DIATOMS. 



The operator may be reminded that the material, even from a 

 considerable number of stomachs, is of course very small in quan- 

 tity, and must be handled carefully ; and as the most beautiful 

 forms are often the lightest, it is of the utmost importance to let 

 the deposit settle thoroughly in the washings of the lighter por- 

 tions of the gatherings. The water holding the diatoms in sus- 

 pension should be allowed to stand at least half-an-hour for every 

 inch of its depth, and hence time will be saved by using watch- 

 glasses and shallow dishes for the purpose. 



A great deal more might be said on the subject of cleaning the 

 diatoms, but it would be a mere repetition of what is found in the 

 authorities on mounting, and it is only experience gained by long 

 practice which will enable the operator successfully to clean mixed 

 gatherings. But, finally, I would advise all enthusiasts on the 

 subject, whether with much or little experience, if they have not 

 tried the sources above mentioned (particularly the stomachs of 

 shrimps), to do so without fail. 



Structure of ©latome^ 



From the " Journal of the Royal Microscopical Society/' 



Vol. v., p. 286. 



DR. G. C. AVallich, writing to the English Mechanic, Vol. xl., 

 p. 496, refers to Flogel's view,* that in such genera as 

 Tricei^atiuni and Coscinodiscus, the little hexagonal or cylindrical 

 cavities, though completely closed by a siliceous film on the 

 internal surface of the valve, are not closed by any such mem- 

 brane on the outer surface of the valve ; and to Cox's, who 

 insistsf that the cellules are closed by a siliceous fihu externally 

 as well as internally. 



Dr. Wallich considers the objections to the latter view insuper- 



* Journal of R.M.S., Vol. IV., p. 665. 

 t Ibid, p. 941, 



