161 



cealed within the thickness of the mantle, and in not 

 adhering to the an'mal by any muscle. This shell is 

 besides very thin, fragile, but slightly concave, and is 

 partially rolled inward on one side: its whorls do not 

 present the conical projection, usually called a spire, 

 or its axis, the part called the columella. The last 

 evolution of the whorl is terminated by the aperture, 

 which is very ample, thin, and expanded at the upper 

 part. Only one species of this genus is known. 



Linn^us had classed this genus with the G. Bulla, 

 which it more nearly approximates than many other 

 shells he had confounded with it; but the peculiarity 

 of this shell, being entirely covered by the animal, and 

 never externally visible, renders its separation very ne- 

 cessary. 



Bullaea aperta. 



BULLA. 



BULLA LI GN ARIA. Linn. 



Enci/cl Meth. pi. 259, / 3, A. B. 



The genus Bulla of Linnaeus included a vast variety of 

 the most opposite genera of shells, which Lamarck 





