86 
BULI.A. 
species might with great propriety be removed to 
other families, still, as the object is not to alter, 
but to elucidate, it is trusted that all the shells 
which can, consistently with the definition, be in- 
cluded in the genus, and all those which have found 
admission rather because they do not belong to any 
other, than that they strictly correspond with the 
character of this Linnsean tribe, will be arranged 
with facility under one or other of the above-men- 
tioned sections. In more than one species of this 
genus the common order of nature seems to be re- 
versed : the shell is enclosed within the mantle of 
the animal, instead of forming an exterior shield. 
So perfectly is it concealed in the B. aperta, for 
instance, that no inexperienced eye would expect 
to find a regular testaceous specimen imbedded in 
the unsightly slug. 
Many of the Bullse are river shells; but the ma- 
rine species are usually imbedded an inch or two 
below the surface of the sand. The name Bulla, 
a bubhUy is very descriptive of the swelled round 
form which characterises the legitimate offspring 
of this family, and should exclude those which 
have been surreptitiously introduced. 
