S6 BULLA. 



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 al! the shells 

 ^vhich 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 Linn;ean 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. aptrta, for in- 

 stance, that no inexperienced eye would expect to 

 find a regular testaceous specimen imbedded in the 

 unsightly slug. 



Many of the Bullae 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 bubhley is very descriptive of the swelled round 

 form which characterizes the legitimate offspring 

 of this family, and should exclude those which have 

 been surreptitiously introduced. 



