56 EVENINGS AT THE MICROSCOPE 



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 cutoff 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 material 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 cor- 

 rugation, 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 nowise possess. The principle is exactly 

 the same in the two cases; but the corrugation of the lime- 

 stone 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 buoy- 

 ant by the large bulk of empty space that intervenes be- 

 tween 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 thig 

 layer projects beyond the inner one, in thin transparent 

 edges, which give us an opportunity of examining their 

 structure, without any artificial preparation. This frag- 

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



