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. Iam 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.} 
* Movos, unus, and rrv722, 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 
