130 THE ELEMENTS 



part of shells seen in collections, are 

 picked up dead on the beach, and are 

 therefore seldom very perfect, as they 

 are tossed to and fro by the waves, 

 and either worn or broken ; and 

 should they even be driven beyond the 

 reach of the tide, in this situation they 

 are exposed to the continual heat of 

 the sun, by which their colours become 

 faded. 



River and land shells are mostly 

 thinner than those of the sea: though 

 this is by no means a general rule, as 

 the Paper Nautilus, some of the 

 Helices, and Pinna?, are extremely 

 thin and brittle. 



Several of the land shells are very 

 beautiful, and elegant in their form, 

 particularly those found in tropical 

 climates In Africa they grow to an 

 amazing size, and would be very un- 

 welcome jjiJests in our house o^ardens, 

 as they commit great havoc among 

 the esculent plants. The Bulla Ze- 



