28 
structure, the question would have been somewhat narrowed, 
inasmuch as the simple form is near to that of Cerithiwm, and 
externally approaches the forms of Chemnitzia and Turritella ; 
but it appears that three of the earliest forms have the most 
complicated internal structure, while the most simple form, 
N. gracilis, does not appear earlier than the succeeding zone. 
The three species are closely allied, and associated with them 
are other species of more simple structure, which are subse- 
quently represented by several species. We must therefore 
infer that an earlier form remains to be discovered. 
It here becomes necessary to consider whether an earlier 
form is represented by N. liassica (Moore,) in short, whether 
that shell is really a Nerinea. It is unfortunate that it is so 
diminutive, only 24 lines in length; its internal structure must 
therefore be difficult to make out with certainty. Moorg, in 
his description of it, says it is slightly folded in the columella. 
It seems to me scarcely possible to be positive on this point, on 
account of the minute appearance of the whorls; the entire 
shell is only one-fifth of an inch in length, and the largest 
whorl is only equal to one-sixth of the whole, or one-thirtieth 
of an inch. Now it very frequently happens that in the 
process of crystallization the internal characters of Nerimea 
become distorted, particles adherent to the interior of the shell 
become crystallized with it, so that projections very like folds 
are constantly observed, and are sometimes found very puzzling. 
If therefore this species is in other respects unlike a Nerinea, 
the slightly folded columella is not, I think, sufficient to 
determine the genus, as the fold may be accidental. Now the 
figure is very unlike that of Nerinea; the aperture does not 
terminate in a channel, as that of the Nerinea usually does, 
and there is no fold on the wall of the shell, but which feature 
is never absent from the true shell. Moore described the 
species from a single imperfect example. It seems therefore 
to be open to doubt whether this is really a Nerinea.* 
= Since the above was written Mr Watrorp has kindly sent me two 
specimens, one of which has been ground down, to show the internal structure. 
There is no fold on the columella or wall, and the shell is undoubtedly a 
Cerithium. 
