46 EVENINGS AT THE MICROSCOPE. 



But now if I give the stage-needle half a revolution, 

 we shall have the horizontal section presented to the 

 eye. In this aspect we acquire much more information 

 as to the structure. The cut has been made very 

 close to one of the horizontal floors, which we see 

 marked all over with a great number of lines, each of 

 which runs hither and thither, in a very sinuous pat- 

 tern. The lines are made up of a brilliant spark- 

 ling substance ; they are in fact the basal portions of 

 what we saw in the other section as thin perpendic- 

 ular plates ; I have cut off the plates close to the 

 bottom, and what we see is their insertion into the 

 floor. 



Thus we perceive that what we took for a multi- 

 tude of plates, were but the various doublings and in- 

 foldings of a single plate of great length, running quite 

 across the floor ; an arrangement by which the strength 

 of the material is greatly augmented. You have often 

 seen the mode in which light walls are made of cor- 

 rugated iron, especially at railway stations, and are 

 doubtless aware that the corrugation, or bending in 

 and out, imparts a strength to it which the mere sheet- 

 iron, if set up as a smooth, plane surface, would in no 

 wise possess. The principle is exactly the same in the 

 two cases ; but the corrugation of the limestone plates 

 in the cuttle-shell is far more perfect than that of 

 the iron ; added to which there is the other advantage, 

 that the aggregate mass of material is made highly 

 buoyant by the large bulk of empty space that in- 

 tervenes between the sinuous folds of the crystal 

 plates. 



It may be interesting to compare with this the 

 structure of the more solid shells of bivalves, which 



