CONCHOLOGY. 95 



FAMILY IV^ 



BuLLACEA. Three genera. 

 1 . Genus Acera. 



Animal. Body ovate, convex, transversely divided above into 

 two parts ; the foot witli dilations in the form of wings below ; 

 head indistinct ; branchiae situated on the back, greatly behind, 

 and covered by a mantle destitute of a shell. Inhabits the Medi- 

 terranean. One species. 



Acera carnosa. 



2. Genus BiiUxa. PL X. 



Animal. Body ovate, somewhat convex above, and divided 

 into two parts transversely ; lateral lobes of the foot very thick ; 

 head indistinct, and without tentacula; branchise placed on the 

 back. 



Shell. Concealed in the mantle, very thin, rolled and spiral 

 on one side ; without a columella and spire ; aperture very large 

 and wide, dilated at the upper part. Inhabits the British seas. 

 Two species. 



Bullaea aperta. BuUsa striata. 



3. Genus Bulla. PI. X. 



Animal. Body oblong ovate, slight convexity ; divided into 

 two portions transverse above, mantle slightly folded posteriorly : 

 visible tentacula ; branchiae dorsal and covered, opening only on 

 the right side. 



Shell. Univalve, ovate globular, convolute, no columella, spire 

 ,not projecting but visible, aperture the whole length of the shell, 

 external margin sharp and .smooth. Inhabits the British and 

 American seas. Nineteen species. 



Bulla physis. Bulla fragilis. 



B. naucum. B. aplustre. 



B. hydatis. B. ampulla. 



B. rugosa. B. lignaria. 



