TO GEOLOGY. 185 



in having folds which are nearly transverse, and occupy 

 almost the whole of the left side of the mouth. 



Three species of Oliva have been observed in Great Bri 

 tain, all being from the London Clay. Thirteen have been 

 observed by M. Deshayes in the Tertiary ; six of these are 

 from the Paris basin. I am not aware of the genus having 

 before been observed in our Formations. 



GENUS MONOPTYGMA.* (nobis.) 



Description. Shell subfusiform ; mouth ovate ; colu- 

 mella furnished with a single fold passing obliquely into 

 the interior of the mouth. 



Observations. The peculiar fold on the columella of this 

 shell at once separates it from any genus heretofore de 

 scribed. In our species, it crosses the columella equidistant 

 from the two extremities of the mouth, in an oblique spiral 

 manner, starting from the base. It is certainly a very in 

 teresting shell, and it is to be hoped that further investiga 

 tion will bring to our knowledge other species. f 



* Mom, unus, and 7r&amp;lt;rvy,f*&, plica. 



t Since the above was written, the arrival of the ship Georgian, from 

 Calcutta, has put in my possession a recent shell which should be 

 placed in this genus. In outline it resembles a Melania, but the single 

 fold on the columella, and the sinus of the base, forbid its being there 

 placed. The lower part of the mouth presents the character of the Ce- 

 rithium. I propose to call it Monoptygma melanoides 

 Y 



