2 INTRODUCTION. 



they lose that brilhancy of external coloring and marking 

 which had characterized their earlier periods. But al- 

 though these differences, in some instances, are sufficient 

 to excite a considerable degree of perplexity, yet there 

 is always a certain appearance by which the Concholo- 

 gist will be able to distinguish the genus and species by 

 the shells alone; for every genus and species has its 

 generic and specific character, either in color, marks, or 

 substance, which the shells of that genus almost invaria- 

 bly retain in all their stages of growth and varieties of 

 form, and consequently are thereby easily knowa and 

 distinguished. 



PROPERTIES OF ANIMALS INHABITING SHELLS. 



These animals possess the power of extending or aggran- 

 dizing 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 endow^ment 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 

 concealed beneath it. 



