A. A. C. ELIOT MERLIN ON NEW DIATOM STRUCTURE. 85 



Auliscus oamaruensis, G. & S. 



The process caps appear as translucent siliceous bubbles covered 

 with a most delicate perforated network, of the type discovered 

 by Mr. Nelson on A. sculptus. 



Auliscus fenestratus, G. & S. ; Oamarn. 



The small process caps of this very beautiful form are finely 

 perforated. It is probably the most difficult of all the varieties 

 of Auliscus, and is a severe test for the finest high-angled 

 apochromats. 



Coscinodiscus oculus iridis, E. 



I have succeeded in obtaining a glimpse of the secondaries on 

 the outer surface crowning the hexagonal framework of this 

 form, which much resemble those of C. asteromphalus in general 

 arrangement, except that the areolations forming the periphery 

 of the rings are very small, and the inner perforations appear to 

 run in parallel lines; but this latter feature may possibly merely 

 be the effect of insufficient optical power. 



All the above-mentioned specimens were examined in styrax. 



Triceratium cancellatum, Grew {-pseudo nervatum); Oamaru. 



Exhibits ;i faint veil of difficult secondaries all over the upper 

 surface of valve. 



Triceratium americanum, var. ; Oamaru. 



Has similar fine structure to the above. 



It has been found that diatomic structure of the kind described 

 in this note is usually best observed with a 12 ocular used in 

 conjunction with the ~-in. aprochromat. The images are 

 generally so excessively faint that no objective yet made will 

 afford sufficient contrast on them to render deep eye-pieces 

 advantageous. It must, of course, be borne in mind that mere 

 closeness of structure is not always necessarily the cause of faint 

 rendering with the best objectives, as perforations in a tenuous 

 transparent siliceous film may well afford so little contrast, with 

 transmitted light, as to be scarcely visible, although they may be 

 sufficiently widely spaced to lie well within the separating power 



