LECTURE XL 159 



ing beyond its usual limits, is capable of destroying 

 the most boasted efforts of human industry. About 

 the year 1730 the most flourishing republic in 

 Europe was made to tremble at the name of this 

 seemingly insignificant creature ; the Dykes of 

 Holland during that year exhibiting such marks 

 of decay in many parts, where they had been 

 attacked by these animals, originally introduced 

 by ships from the East Indies, that great appre- 

 hejasions were entertained of the Dykes giving way, 

 and exposing the country to the ravages of the 

 ocean. 



The last Linnaean genus of the Univalves, the 

 Sabelltt, is improperly placed among the shelly 

 tribe; since the tubular structure, by Linnaeus 

 called the shell, is merely composed of aggluti- 

 nated grains of sand, lined by a connecting mem- 

 brane. The inhabiting animal is allied to the 

 genus Nereis among the naked Mollusca. 



Proceeding to the Bivalve Shells, we shall ob- 

 serve that the chief instances in which the inhabit- 

 ing animal differs in character from the rest, are 

 those of the genera entitled Anomia and Pinna. 

 Of these the genus Anomia is inhabited by an ani- 

 mal whose nature is not yet fully ascertained; and 



