MOLLUSCA. 63 



the direction of the whorls been confined to one individual 

 or two, of particular species. But when we observe all the 

 individuals of particular species, nay even of genera with 

 their whorls thus invariably reversed, we are disposed to re- 

 gard the occurrence as connected with the primary struc- 

 ture of the animal, and not as the result of accident. Be- 

 sides, the viscera of the animal of a reversed shell are not 

 placed in the same position in relation to its back or belly, 

 as the animals of the dextral species. A simple change of 

 direction in the spire, therefore, will not convert a dextral 

 into a sinistral species, and the character must be consider- 

 ed as of a higher order than those employed for the separa- 

 tion of the species merely. We consider sinistral shells as 

 belonging to distinct genera from those which are dextral, 

 it being inexpedient to make use of the character for higher 

 divisions. 



Among many of the univalves, the animal is furnished 

 with a lid, by means of which it can close up the entrance 

 of the shell after it has withdrawn itself into the cavity. It 

 is in general corneous, sometimes also calcareous. It is 

 usually flat, and attached to the superior and posterior part 

 of the foot of the animal. The shells which possess this lid 

 are generally termed operculated shells. They must not 

 be confounded with those land shells of which the animals 

 form a temporary covering to the mouth, previous to winter, 

 for the purpose of protecting them from the vicissitudes of 

 the weather. This lid, in the former case, is permanent, in 

 the latter deciduous ; in the former it adheres to the ani- 

 mal, being in connection with it ; in the latter only to the 

 margins of the mouth of the shell. 



This character was first employed by Adanson in the con- 

 struction of the second section of his class univalves, and 



