428 PHYSICAL GEOGRAPHY. 



hexagon. In fact, such a collection of bubbles, thus cut in halves, looks 

 very much like the transparent siliceous membrane of a diatom. It is 

 impossible, however, to give any idea in words of the beauty of the 

 diatomaceae, and, in fact, the best of illustrations affords but a faint notion 

 of them ; they must be seen and closely observed to be fully appreciated. 

 Their great interest to all students of nature will perhaps be understood 

 from a perusal of this brief sketch, wherein their principal points of 

 structure, habits, modes of occurrence, and uses, are set forth in such a 

 way, it is hoped, as will be readily understood, and at the same time 

 prove interesting to others than those intending to be students of these 

 organisms. 



Having now arrived at a pretty clear idea of the typical structure of 

 diatoms in general, let us make ourselves more thoroughly acquainted 

 with some of the various forms in which they make their appearance. 

 If we take, as a representative of the usually free circular or discoid dia- 

 toms, the genus Aulacodiscus, we find the valves perfectly round in out- 

 line, and usually only slightly convex near the margin, the concavity for 

 holding the cell-contents being thus somewhat flattened, its sides being 

 limited as usual by the connecting membrane or zone, which in this 

 genus is narrow. The structure of the siliceous material which goes to 

 make up the valve is of the following character: On the exterior is a 

 plate marked with coarse hexagons, which really are only net-like ; that 

 is to say, they merely constitute a hexagonal framework of boxes without 

 tops or bottoms, set side by side, and arranged more or less regularly, so 

 as to radiate from the centre to the edge of the valve. The radiant 

 arrangement of these coarse markings varies in regularity in the differ- 

 ent species of the genus, but in all is apparent. Inside of this coarsely 

 marked plate is another, so that the large hexagons have the character of 

 honey-comb ; that is to say, the sides are perpendicular to the flat surface 

 of the inner plate, which thus constitutes for them a bottom. And this 

 inner plate is commonly, although not always, constructed after the same 

 manner as the outer one, being set all over with small hexagons, which 

 are so small that as yet their character has not been studied, but most 

 likely it is similar to the larger ones, that is to say, being honey-comb- 

 like ; but its hexagons probably have not in their turn still smaller mark- 

 ings within them. At any rate, in most species of Aulacodiscus, both 



