182 MOLLUSC A. 



MRS. F. 



Not so, for conchology implies the knowledge of shells; 

 but all mollusca have not this kind of covering. Those 

 which have, are called testaceous, from the Latin testa, a 

 shell. Conchology, therefore, may be defined as the classi- 

 fication of the testaceous mollusca, according to the forms of 

 their shells, and is consequently a very distinct science from 

 that of the knowledge of molluscous animals. 



MARY. 



Pray, mamma, what are shells made of? 



MRS. F. 



Like the bones of vertebrated animals, they are composed 

 of a calcareous substance, almost as heavy and as hard as 

 marble, which substance is produced in layers. In propor- 

 tion as the animal increases in age, it forms a new layer on 

 the inner surface of the shell, which extending beyond the 

 edge of the preceding layer, increases the size of the shell in 

 length, breadth, and thickness. 



HENRIETTA. 



But how can naturalists ascertain this point? 



MRS. F. 



By comparing the shells of the same species of animals of 

 different ages; as for example, the mussel, in which the old 

 shell is seen to be composed of many distinct layers; whereas 

 that of the young animal only consists at first of one stratum, 

 not soft and gelatinous, but equally firm as the old shell, 

 though, of course, more fragile, on account of its thinness. 



ESTHER. 



Then slugs and the cuttle fish, for instance, are both mol- 

 lusca, although they would not be mentioned in a work on 

 conchology. 



'\ :. 



MRS. F. 



Yes ; but the last contain a calcareous substance within 



