269 



Short Note on the Fixer Structure of certain Diatoms. 

 By E. M. Nelson and G-. C. Karop. 



(Read March 26th, 1886. J 



Plate XVII. 



On examining certain Diatoms with the finest oil-immersion 

 objectives, and under conditions of illumination snch as are 

 absolutely essential if the full aperture, and, therefore, resolving 

 power, of these glasses is to be utilized, some details of struc- 

 ture are brought into view which are otherwise quite invisible, 

 and, as far as we know, have not hitherto been correctly 

 described or properly figured. Acting on this belief we have 

 ventured to bring before your notice some short observations, 

 accompanied by careful drawings, recently made on a few well- 

 known forms. 



1. Coseinodiscns ash rom/phalos. This diatom, although con- 

 sisting of a single siliceous membrane, has a double structure, 

 viz., coarse and fine areolations, the latter within the former. 

 The coarse areolations are for the most part circular in outline, 

 and the intervening silex is thick. Inside these areolations is 

 a most delicate perforated membrane, the outermost row of per- 

 forations being much larger than the rest. This membrane is 

 so thin and fragile that it is often broken out, and when this is 

 the case the coarse areolations appear to have a crenated edge 

 (PL XVII., Fig. 1). 



2. Isthmia nervosa. This is similar in construction to the 

 above, having a single membrane with a two-fold structure, a 

 fine perforated membrane inside coarse areolations. The coarse 

 areolations in this diatom are very large, and the silex corres- 

 pondingly thick. At the same time the inner membrane is 

 excessively' thin and delicate as in asteromphalos. The perfora- 



