42 EVENINGS AT THE MICROSCOPE. 



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 pattern. The lines are made up 

 of a brilliant sparkling substance; they are, in fact, the 

 basal portions of what we saw in the other section as thin 

 perpendicular 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 multitude of 

 plates, were but the various doublings and infoldings of a 

 single plate of great length, running quite across the 

 floor : an arrangement by which the strength of the ma- 

 terial is greatly augmented. You have often seen the 

 mode in which light walls are made of corrugated iron, 

 especially at railway stations ; and are doubtless aware that 

 the corrugation, or bending in or 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 

 intervenes 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 have been so 

 elaborately studied by Dr. Carpenter. In general, these 

 consist of two very distinct layers, well seen in the valve 

 of the Pearl Oyster and its allies. The Pinna, or Wing- 

 shell, the largest of our native bivalves, affords us a good 

 example, especially of the external layer, since here this 

 layer projects beyond the inner one, in thin transparent 

 edges, which gives us an opportunity of examining their 

 structure without any artificial preparation. This frag- 

 ment, taken from the edge of one of those leafy expan- 

 sions, we will examine with a low magnifying power. Each 



