ON THE FINER STRUCTURE OF CERTAIN DIATOMS. 43 



this beautiful form has a still more minute tracery inside each 

 areolation. How truly marvellous this is when we consider 

 that the coarse areolation only measures '0000727 inch in 

 diameter. 



11. Aulacodiscus Kittonii. Eight rays. Peru. This diatom 

 does not resemble the A. Sturtii in the peculiarity mentioned 

 above, but follows the usual type of diatoms in having its 

 secondary pattern inside the primary. Delicate structure will also 

 be noticed between the coarse areolations ; this specimen, there- 

 fore, is a sort of compromise between the peculiar Aulacodiscus 

 structure and the usual type. It is very delicate, but not so small 

 as No. 10, the large areolation measuring "0000798 inch. 



The objective used for the work in this and our former paper 

 was an achromatic oil immersion, T \, 1. 43, N.A., by Powell. The 

 magnification generally adhered to was 1200 diameters. 



