SHELLS AND THEIR ANIMALS. 211 



want of water, keep their shells open, this cunning 

 animal may be seen stationed on the sea shore, but 

 aware of the danger of inserting his paws between 

 the valves, he first drops in a stone or two, by 

 which he prevents their closing, and is then ena- 

 bled to extract his prey with impunity. Not only 

 are Testaceous animals useful in various ways, 

 their shells also are valuable. In America the 

 greatest quantity of the lime used for agricultural 

 and architectural purposes is made of calcined 

 shells. The public streets of Christianstadt and 

 Santa Cruz are paved with the Strombus Gigas. 



Not only is lime thus artificially obtained from 

 shells, but a very important use of Molluscous 

 animals is the conversion through their agency 

 of other materials into lime. Strata of Limestone 

 some miles in extent and many feet deep are com- 

 posed almost wholly of Shells. In the marble 

 used in Chimney pieces may often be detected 

 the forms of various shells. 



In uncivilized countries the natives delight to 

 deck their persons with shells, and where the 

 working of iron is still unknown, they often em- 

 ploy shells as its substitute, and convert them into 

 agricultural or domestic implements, also into 

 warlike instruments ; and in the South of China 

 and India, thin layers of some large flat shells, 

 when polished, are used for windows instead of 

 glass. The military horn of many of the African 

 tribes is formed of the Murex Tritonis. The blue 

 and white belts of the Indians of North America, 

 P 2 



