CONCLUDING OBSERVATIONS. 365 
the size of the head of a small pin, may sometimes be | 
seen thus walking on the glass walls of an aquarium; and 
a great variety of species can be found in the sand of 
most sea-coasts, as any one may see if he examine a hand- 
ful of it with a pocket lens. In the chalk formations 
there are remains of even smaller animals than these. 
Ehrenberg, on examining chalk very minutely divided, 
found in it some many-chambered shells, some of them 
whole and some in fragments. He calculated that there 
were a million in every cubic inch, or ten millions in ey- 
ery pound. He was able to discern them even in the 
glazing of a visiting card, although the chalk in this case 
had been subjected to such minute division that one 
would suppose all trace of organization to have been lost. 
636. The earth in and about the city of Richmond, 
Virginia, is filled with various shells of Animalcule. <A 
portion of one of these shells, as seen through a power- 
ful microscope, is given in Fig. 276. There are various 
species of this shell, called, very appropri- 
ately, Coscinodiscus (sieve-like disk), va- 
rying in size from the one hundredth to 
the one thousandth of an inch in diame- 
ter. The guano brought from the island 
of Ichaboe is found to contain multitudes 
of this and other shells, making a beauti- 
ful display as a little of the dust is placed 
in the field of the microscope. These 
shells are the remains of animalcule that 
lived in the water and were eaten by fish- 
es. Then these fishes were devoured by 
sea-birds; so that these shells must have 
Fig. 276, passed through the process of digestion 
twice, and after that were exposed in the guano-bed to 
the ordinary causes of decay perhaps for centuries; and 
yet, says Professor Brocklesby, “ under all these influ- 
ences they remain unchanged, and the eye of the natural- 
ist at last detects these minute structures, still possess- 
a 
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