10 INTRODUCTION. 



differences, in some instances, are sufficient to excite a 

 considerable degree of pei*plexity, yet there is always in 

 every shell a definite character, by which the Conchologist 

 will be able to distinguish its genus and species ; for each 

 genus or species is characterized by structure, substance, 

 marks, or colour, which the shells of that genus or species 

 almost invariably retain in all their stages of growth and 

 varieties of form, and consequently are thereby easily known 

 and distinguished. 



PROPERTIES OF ANIMALS INHABITING SHELLS. 



These animals possess the power of extending or ag- 

 grandizing their calcareous habitation or shell, and are 

 also enabled to repair whatever breaches their brittle dwell- 

 ings may have sustained by the turbulence of the ocean. 

 The operation of enlarging and repairing the shell is sup- 

 posed to be effected by a peculiar endowment of the ani- 

 mal, which can at pleasure discharge a viscous humour, 

 that soon becomes hardened or consolidated, and in time 

 acquires the consistence of the rest of the shell. 



Many shells are covered with a cutaneous or skin-like 

 substance, called the epidermis, which serves to protect the 

 beautiful colors and markings which are frequently conceal- 

 ed beneath it. 



CUSTOMARY USES TO WHICH SHELLS AND THEIR INHABITANTS 

 ARE CONVERTED. 



The principal benefit derived by man from shell-fish 

 appears to be in the way of food. Whole countries are 



