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has removed from it, and classed separately in natural 

 groups, under the names of Bullaea, Ovula, Achatina, 

 Physa, Terebellum, and some are comprised in the G. 

 Pyrula, and G. Bulimus. The great confusion that for- 

 merly existed, and the manifest incongruity of blending 

 together marine, fresh water, and land shells, which 

 may be observed in the LinnaeanG, Bulla, is now elu- 

 cidated; and, by this arrangement, a very natural as- 

 sociation is formed of each different species. The 

 shells constituting the present G. Bulla, as it is defined 

 by Lamarck, possess one consistent family character, 

 viz. shell univalve, of a globose oval form, rolled up, hav- 

 ing no columella, and without a spire; the large exterior 

 whorl elevated above the others, giving an umbilicated 

 appearance to the upper part of the shell; aperture open 

 the whole length of the shell, and generally wider at the 

 base; the outer edge sharp and smooth. 



The animal inhabiting the Bulla can altogether re- 

 cede within its shell, and therefore is very distinct from 

 that of the preceding genus, in which the shell is quite 

 enveloped by the mantle of the animal. The thickness 

 and coloring matter of the Bullae, as well as their more 

 regular convolutions, and the animal being attached by a 

 muscle to its shell, all clearly tend to render the forma- 

 tion of the two genera necessary. 



