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of the shell after it has withdrawn itseif into the cavity. [t 
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 
