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 



* Metro;, unus, and vrvy^ct^ 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 Mclania, 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 



