172 MOLLUSCA FROM THE CRAG. 



Bulla,* Kl^ein, 1753. 



Atys. Montf. 1810. 



Gen. Char. Shell ovate or cylindrical, the last whorl sometimes entirely enveloping 

 the preceding ones ; convulute upon a nearly horizontal axis, with a spire generally 

 visible, but always short ; aperture the length of the shell, efifuse at the lower part, 

 and narrow above ; often striated or ornamented on the exterior, sometimes smooth 

 and glossy ; texture thick, opaque, corneous or subhyaline ; outer lip sharp and 

 smooth. 



Shells belonging to this genus, but possessing a visible and slightly elevated spire, 

 have been separated from it and united into a genus, under the name Bullina ; such 

 characters, depending merely upon the deviation from the horizontal form of volution, 

 or rather upon the elevation or depression of the outer lip, are not satisfactory, and the 

 gradation from the visible and elevated spire, to the truly spindle-shaped form produced 

 by the elevation and acumination of the upper part of the outer lip is imperceptible. 



The animal, it is said, is remarkable for not having antennae, and for the foot being 

 laterally expanded, so as to be used in the way of fins, to swim with, and the mantle 

 does not include the shell. 



Many of the species are found in shallow water, while others have a considerable 

 range in depth, and some are frequenters of estuaries. 



Mr. Bean appears to have found this genus in the combrash, and one is also 

 figured in the ' Transactions of the Geological Society' as from the Wealden formation. 

 Numerous species are known in the Eocene period, and it is widely extended as a 

 recent genus, being found in all parts of the world, and not restricted to any climate. 

 Those found in the Crag, identical with existing species, are either British or Northern 

 forms. 



* Etym. A bubble. 



