THE KOMANCE OF NATURAL HISTORY. 



is much wider than it is deep, will give a good 

 idea of many of the Diatoms, such as Arachnodis- 

 cus. The top and bottom of the box are formed 

 by flat circular glassy plates, called valves, and 

 the sides by a ring or hoop of similar material. 

 Sometimes the outline of the valves (with which 

 the hoop agrees) is oval, or oblong, or square, or 

 triangular, instead of circular; and their surface is 

 sometimes convex in various degrees, but the 

 side is generally upright, or in other words, the 

 surface of the hoop passes in a straight line from 

 the edge of one valve, whatever its outline, to 

 that of the other. 



Here then is a box formed of pure transparent 

 flint-glass, very thin and delicate, and very brittle. 

 The valves are marked with minute dots, which 

 appear to be either knobs or pits; or with lines, 

 either depressed or raised. In the beautiful Arach- 

 nocliscus, both of these modes of sculpturing are 

 present. Each valve is marked with a number of 

 most delicate lines, which radiate from a central 

 circle of dots to the circumference; these radii are 

 connected by a multitude of cross lines, bearing 

 the closest resemblance to the elegant webs spun 

 by our common geometric spiders, whence the 

 name given to the genus; while in the spaces 

 marked out by these reticulations there are rows 

 of minute round dots. Altogether, the effect of 

 this complex pattern of sculpture is most charm- 

 ing, and is heightened by the brilliant translu- 

 cent material in which it is wrought, which, 

 as has already been observed, is like the purest 

 glass. 



During life there is, in every individual, a small 

 round body in the centre of the enclosed cavity, 

 called the nucleus, and this is surrounded by ir- 

 regular masses of yellowish substance, called the 

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