H. MORLAND ON DIATOM STRUCTURE. 305 



valves soon showed me that the structure was of the most complete 

 kind, according to Dr. Van Heurck's views. Most young micro- 

 scopists looking at a valve with the outer surface uppermost under 

 a lowish power, say a -fa inch objective, would regard the markings 

 as solid dots, but I could soon convince him to the»contrary by show- 

 ing him a valve under this low power, wherein the balsam has 

 failed to duly penetrate several of the cells, the valve showing 

 bubbles of air most distinctly in the body of the structure, each 

 bubble being contained in a separate cellule. So much for the solid 

 dot theory ; however, this is only one step towards getting a proper 

 knowledge of what we are examining. I have another slide on 

 which are two broken valves, one with the " inner," the other with 

 the " outer ' ; side uppermost. I have mounted these valves thus, 

 as I find that when using high powers it is better to have the sur- 

 face which is being examined uppermost, and not with any structure 

 above it, which interferes considerably with proper definition. 

 On examining the " inner '" surface, I find that it has a small per- 

 foration with thickened edge in the centre of each larger cell in the 

 structure above (or rather underneath as examined), but on examin- 

 ing the " outer" surface on the other valve the "dots" appear 

 circular, but not so definitely as to enable me to decide exactly 

 upon their nature ; however, I have yet another slide of this 

 diatom, with fragments only of the valve mounted in various positions 

 and one of these fragments is broken in such a manner that I can 

 examine the inner surface, interior structure, or outer surface 

 separately, which enables me to give the following description of 

 the structure of this valve : — The inner surface consists of a layer 

 with small perforations, some y^-go" mcn across, with thickened 

 borders (in fact, I find thickened borders and rounded edges very 

 common to perforations) ; above this is a framework of tubes (one 

 to each perforation below), with more or less straight-sided borders. 

 Some of these tubes have 4, 5, or 6 sides, and they are by no means 

 regular in shape. As the outward surface is approached the angles 

 get filled up, and the irregularly shaped cells end in circular open- 

 ings rather smaller than the cells themselves. 



The next diatom I shall describe is Arachnoidiscus Ehrenbergii, 

 and here the structure is totally unlike those of which I have 

 already been speaking. This valve is composed, firstly, of an outer 

 layer, with large, slightly angular perforations, radiating from the 

 centre to the circumference. That these markings are perforations 



