SHELLS. 53 



porary covering, which adheres to the sides of 

 the shell, and is deciduous, that is, falling after 

 a certain period, from Latin decido, I fall. The 

 operculum of the marine shell is of a very differ- 

 ent nature. It is either a calcareous or horny 

 substance, permanent, and not attached to the 

 shell, but to the foot of the animal, who draws it 

 over the mouth when he recedes into his abode. 

 Now tell me the position of the mouth. 



Child. It is on the right side of the shell. 



Teacher. Observe whether that is invariably 

 its place. 



Child. No, here is one shell in which it is on 

 the left side. 



Teacher. This is the case in some few species, 

 and the shell is then said to be sinistral, a word 

 derived from the Latin sinistra, the left hand. 

 The others are called dcxtral shells, from dextra, 

 the Latin for the right hand. The sinistral 

 shells are also said to be reversed, because the 

 whorls proceed in a direction contrary to their 

 usual course. Now look at this very singular 

 shell. (Murex haustellum.) 



Child. It has a long tube. 



Teacher. From what part does it proceed ? 



Child. From the mouth. 



Teacher. Do you recollect any animals that 

 have a somewhat similar projection proceeding 

 from their rnouths ] 



Child. Oh yes ; birds their beaks are like 

 this part of the shell. 



Teacher. This projection in shells is also 

 called a beak, or rostrum, the Latin for beak. 

 5* 



