170 
wards the centre, and almost disappear near the edge of 
each whorl, where smaller ridges commence ; there are 
about five small ridges to each of the large ones. Upon 
the outer whorls of large shells, the smaller ridges are 
wanting, so that fragments of the same individual appear 
like distinct species, whence I have named if decipiens. 
This is found in the same kind of Clay as A. biplex, 
and, like it, often forms the nucleus of a Septarium. I have 
part of a very large one from Pakefield, near Lowestoft 
in Suffolk, given to me by my old botanical Friend, 
Dawson Turner, Esq. The fragments figured are from 
the inside of a decomposing Septarium, and are in such a 
state that they frequently break straight through instead 
of in the direction of the septa of the Shell. They are 
from the North side of Highgate Hill. 
