LAMP-SHELLS AND THEIR ALLIES 11 



are more common and more heavy in limestone re- 

 gions than in those provinces where lime is deficient 

 in quantity. Most rivers are constantly carrying 

 dissolved limestone to the sea, where it is used not 

 onh^ by mollusks for the making of their shells, but 

 also in vast quantities by coral animals, crabs, worms, 

 and many other creatures, as well as by certain forms 

 of plants. By geological changes the coral reefs and 

 shell banks may be lifted out of water and trans- 

 formed again into beds of limestone. 



Naturalists do not wholly agree as to the limits 

 which divide the great class of mollusks from other 

 animals. Some of the lowest of these creatures have 

 affinities with the worms and the sea-bottles, but 

 these matters may be studied more fully by those 

 who are specially interested in the subject. For the 

 present we may assume the usual division to be cor- 

 rect, and will describe the shells to be found on the 

 seacoast under four heads, namely, the lamp-shells, 

 the bivalves, the sea-snails and the cuttles, of which 

 the second and third divisions include the vast ma- 

 jority of specimens. In fresh water lakes and rivers 

 there are bivalves and pond-snails, but no lamp- 

 shells or cuttles, while on the land are found only 

 air-breathing snails and slugs. Thus we see that 

 the first and last of the four great divisions are con- 

 fined to the ocean, the second to water either salt or 

 fresh, while the snails, also called univalves, are the 

 most widely distributed ot all, living alike in air, 

 fresh water, and also in the sea. 



To be sure the same animal cannot endure a 

 change from one medium to another, and a trained 



