172 MOLLUSCA FROM THE CRAG. 



Bulla,* Klein, 1753. 



Atys. Mont/. 1810. 



BULLUS. 



Id. 



Rhizorus. 



Id. 



Scaphander. Id. 



Aplustrum 



. Schum. 1817. 



Assula. 



Id. 



Hydatina. 



Id. 



Naucum. 



Id. 



DlAPHANA. 



Brown, 1827. 



Retusa. 



Id. 



VOLTARIA. 



Id. 



Cylindrella. Swains. 1840. 



BULLINA. 



Ferrusac. 



Cylichna. 



Loven. 1846. 



Amphysphyra. Id. 



AlilCULA. 



Ehrenb. (ex Loven.) 



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

 enveloping the preceding ones ; convolute upon a nearly horizontal axis, with a 

 spire generally visible, but always short ; aperture the length of the shell, effuse 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 satis- 

 factory, 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 antennee, 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 cornbrash, and one is also 

 figured in the ' Transactions of the Geological Society' as from the Wealden forma- 

 tion. 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. 



