MOLLUSCA OR UNIVALVES. 253 



it has attained its full growth, the outer lip 

 is thickened and rolled inwards, the back 

 marked with the most beautiful patterns, and 

 bearing an exquisite polish, and the spire, at 

 the same time, if not entirely hidden, scarcely 

 projects. The animal itself undergoes a con- 

 siderable change in appearance during its 

 growth. Its mantle is at first small, but in- 

 creases with the enlargement of the shell, and 

 expands at its sides into two large wings ; 

 from these is deposited the final layer of shelly 

 matter which forms the exterior coating, and 

 which is caused by the animal enveloping the 

 shell entirely in its mantle. At the point 

 where the two expansions of the latter meet 

 on the back of the shell a line is consequently 

 formed, which is called the dorsal line ; but 

 occasionally these lobes are so much extended 

 as not only to meet, but to wrap over each 

 other, in which case there is no line. 



This shell is generally found in the seas of 

 hot climates, the Indian Ocean, and the seas of 

 Africa and America. A few species are met 

 with in temperate regions ; but they possess no 



