156 THE MINUTE. 



the deposition of flinty matter, become glassy valves,* cor- 

 responding to the two outer valves, and then the whole 

 frustule separates between these two new valves, and 

 forms two friistules. The old hoop (in some cases at 

 least) falls off, or allows the hoops of the new-made frus- 

 tules to slip out of it, like the inner tubes out of a tele- 

 scope. 



Now, the sejDaration of the frustules thus made is not 

 always so complete, but that they remain adherent to one 

 another, by some point of contact ; and hence arises a 

 most singular and interesting appearance often presented 

 by these bodies. Let us suppose that the original frustule 

 was of the shape of a brick, and that by successive acts of 

 self-division, it has formed itself into a number, say a 

 dozen of bricks. These, of course, are laid one on another, 

 forming a pile ; but all the individuals adhere to one 

 another by a minute point at one corner, and the matter 

 of adherence is sufficiently tenacious and sufficiently 

 yielding to allow of the brick-shaped frustules moving 

 freely apart in every point, except just the connecting 

 angle. It is not the same corner that adheres all up the 

 23ile; more frequently opposite corners alternate with each 

 other, yet not very regularly, and thus an angularly jointed 

 chain of the little bodies is formed, which is very charac- 

 teristic. In some species, in which the form is a lengthened 

 oblong, the frustules have the faculty of sliding partially 

 over each other, and thus the chain resembles a series of 

 long steps. 



Sometimes the frustules, perhaps of a graceful wedge- 



