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 ev- 

 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 Animalculae. 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, makiijg a beauti- 

 ful display as a little of the dust is placed 

 in the field of the microscope. These 

 shells are the remains of animalculae 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- 



