DESMIDS, DIATOMS, AND FRESH-WATER AhGM. 87 



edge of the- lower and usually younger valve. The 

 •entire box-like diatom is called the frustuie; the sur- 

 faces of the upper and lower valves are usually marked, 

 and usually but not always shaped, alike, aud are called 

 the sides. But when the frustule happens to be 

 turned so that the narrowest part, or that part corre- 

 spoiiding to the thickness of the pill-box, and called 

 the front, is towards the observer, then the shape is 

 so different from that of the valves as often to puzzle 

 the beginner. If in doubt about the position, gently 

 tap the cover-glass with a needle, when the frustule 

 will generally roll over on its broad side. All this 

 seems somewhat bewildering at first, but there will be 

 no difficulty if it is borne in mind that the thickness 

 of the pill-box corresponds to the /ronf of the frus^u/e, 

 and the broad surfaces of the lid and bottom to the 

 sides of the valves. 



In addition to the valves which together form the 

 frustule, there is another part, of which less is heard 

 among diatomists, although it is important and is it- 

 self sometimes striated or sculptured. This corre- 

 sponds to that portion of the pill-box remaining after 

 the top and the bottom have been removed. The part 

 of the box therefore corresponding to the height or 

 thickness of the diatom frustule, is called, in the dia- 

 tom, the hoop, and after the frustule has been cleaned 

 and the valves separated, the hoop may bften be seen 

 lying in the mounting medium, frequently set up on 

 its edge, and still retaining somewhat of the contour 

 of the original frustule. These hoops are sometimes 

 rather puzzling when first found in a mounted prepa- 

 ration of diatoms, as they generally resemble very 

 narrow, curved or rounded filaments which may lead 



