ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 661 



with only rudimentary spines. The endochrome in each of the 

 frustules is disposed as it was in the original frustule before division 

 began. As growth goes on in the new valves they become thicker, 

 and separate from each other further and further, being apparently 

 pushed apart by the growth of the lengthening spines, until finally 

 they are sometimes half as far apart as the width of the original 

 frustule. The hoop of the parent frustule goes on widening to 

 accommodate this growth, until, when the two new frustules are 

 completely grown, they are ready to separate, and to repeat the process 

 each for itself. One or both of the frustules now drops out of the 

 hoop, and it is not uncommon to find the wide hoop with one frustule 

 attached and one gone. The new valve is in this species clearly 

 formed by the deposition of silex in or upon a membrane previously 

 formed, and not by growth along an edge. 



Finer Structure of certain Diatoms.* — Messrs. E. M. Nelson 

 and G. C. Karop find that on examining certain diatoms with the 

 finest oil-immersion objectives, and under conditions of illumination 

 such as are absolutely essential if the full aperture, and therefore 

 resolving power, of these glasses is to be utilized, some details of 

 structure are brought into view which are otherwise quite invisible, 

 and, as far as they know, have not hitherto been correctly described 

 or properly figured. 



1. Coscinodiscus aster omphalos. This diatom, although consisting 

 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 perforations 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. 



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

 having a single membrane with a twofold structure, a fine perforated 

 membrane inside coarse areolations. The coarse areolations in this 

 diatom are very large, and the silex correspondingly tbick. At the 

 same time the inner membrane is excessively thin and delicate as in 

 C. asteromplialos. The perforations are large and irregular in shape 

 around the margin, but smaller and circular in the centre. A 

 broken areolation is figured to show the fracture passing through the 

 perforations. 



3. Triceratium favus. This diatom is very similar to the preceding. 

 The coarse areolations are hexagonal in form and very deep. At the 

 bottom of these is a delicate perforated membrane, the perforations 

 being circular and arranged for the most part in rows. A figure is 

 given showing a fracture passing through the minute perforations, the 

 resolution of which may be considered one of the most crucial tests 

 for the Microscope of the present day. 



4. Eupodiscus argus. This diatom differs from the above, inas- 



* Journ. Quek. Micr. Club., ii. (188G) pp. 269-71 (1 pi.). 



