OF CONCHOLOGY. 5 



lobsters, sea-urchins, &c. are of the 

 crustaceous order. 



They differ in their composition 

 from the testacea, the calcareous part 

 of their shell being phosphate of lime, 

 in place of the carbonate of lime. 



The Crustacea differ from the testa- 

 cea in a still more striking point of 

 view : it is well known that crabs, 

 lobsters, &c. cast their shell or cover- 

 ing annually ; whereas the testaceous 

 worms retain it as long as they exist. 

 Besides the shells of crustaceous ani- 

 mals, seem to grow all at once ; those 

 of testaceous, evidently are made by 

 the animal, gradually adding to them 

 either annually or at least periodically; 

 as may be distinctly seen in the com- 

 mon muscle ; and all shells are com- 

 posed of layers, which is not the case 

 with crabs, &c. This may be easily 

 proved by taking a shell and filing it 

 slowly, when the different layers are 

 very perceptible ; or put a muscle- 



