THE STORY-BOOK OF SCIENCE 



"Here is another which, like the preceding, is 

 found in the Mediterranean. It is the spiny mollusk. 

 The creature that inhabits it produces a violet glair, 

 from which the ancients derived, for 

 their costly stuffs, a magnificent color 

 called purple." 



"How are shells made I" asked Claire. 

 "Shells are the dwellings of creatures 

 called mollusks, the same as the spiral 

 snail's shell is the house of the horny 

 little animal that eats your young flow- 

 ering plants." 



"Then the snail's house is a shell, the same as the 

 beautiful ones you have shown us," Jules observed. 

 "Yes, my child. It is in the sea that we find, in 

 greatest number, the largest and most beautiful 

 shells. They are called sea-shells. To these belong 

 the helmet-shell, cassidula, and spiny mollusk. But 

 fresh waters, that is to say streams, rivers, ponds, 

 lakes, have them too. The smallest ditch in our 

 country has shells of good shape but somber, earthy 

 in color. They are called fresh- water shells." 



"I have seen some in the water resembling large, 

 pointed, spiral snails," said Jules. "They have a 

 sort of cap to close the opening." 

 "They are paludinidae. " 

 "I remember another ditch shell," 

 said Claire. "It is round, flat, and as 

 large as a ten or even twenty-sou 

 piece." 



"That is one of the planorbinae. Finally, there 

 are shells that are always found on land and for 



