xlii INTRODUCTION. [CH. 



there can scarcely be a question that these moUusks 

 have abundance of time to effect their purpose. It is 

 said that even the hardest marble is not proof against the 

 softest impressions, and that the big toe of St. Peter's 

 statue in the Vatican has been nearly worn away by the 

 lips of fair devotees. The osculatory process is not un- 

 like that of suction. 



Occasional appearance and disappearance. — Both sea 

 and land furnish instances (some of which are difficult 

 to explain) of the periodical appearance and disappear- 

 ance of certain species of Mollusca in particular places. 

 Their arrival and departure are often sudden and seem- 

 ingly capricious. In the case of marine species, this 

 phenomenon is probably the result of changes in the 

 course of tidal and other currents, as well as of the 

 migratory habits of fish. These currents, by accumu- 

 lating or removing deposits of mud, sand, and gravel, 

 wliich afford shelter and food to Mollusca, conduce 

 greatly to their congregation or dispersal. When such 

 deposits are rapidly formed, the shell-bed becomes co- 

 vered up or silted; and the Mollusca are entombed 

 alive for the benefit of future geologists. When their 

 chief enemies, the fish, desert their former quarters and 

 migrate to another feeding- ground, the Mollusca then 

 increase and multiply, being unthinned except by the 

 tigers of their own kind, or occasionally by the curious 

 conchologist, or by all-devouring death. The destruc- 

 tion of shell-beds by marine currents may account for 

 the prevalent notion that some parts of our sea-coast 

 (as for example South Devon), which used to yield such 

 regular and plentiful harvests of shells to collectors, 

 are now scarcely worth searching, — -it being said that 

 the shells have " deserted" the coast. The unexpected 

 occurrence of some kinds of land and freshwater Mol- 



