194 THE MICROSCOPICAL NEWS. 
in great abundance. ‘This species is known to inhabit only brackish 
water, and besides these shells some diatoms which I have found 
in the mud bank are really other than fresh-water species. Mr. 
Rimmer writes thus :—‘‘ The question, which is a very interesting 
one, arises, as to how these molluscs, Aydrobia ventrosa, came 
to be in a spot so far removed from ‘ brackish water,’ which is their 
proper habitat. My own idea is that at some period the tide must 
have flowed up the Avon as high or perhaps higher than the city, 
and that in course of time an accumulation of debris carried down 
by the stream gradually formed a barrier which slowly but 
effectually checked its progress. Other causes, however, may have 
been at work; in any case the subject is, I think, quite worthy 
of the attention of your local geologists.” 
It now only remains for me to bring before you the contents of the 
mud bank I have described. The Mollusca form a sub-kingdom of 
the animal world, divided into—1, Acephalous without a head; 2 
Cephalic having a head ; the former division have Bivalve, the latter 
Univalve shells, and of the latter only I have to speak, because I have 
discovered no bivalve shells in the deposit of the bath. The 
Cephalic molluscs, Mr. Rimmer writes, “ are of a higher organism 
than the Acephela, their nervous system is more fully developed, 
they have a distinct head, and usually tentacles or feelers on 
the tips, or sometimes at the base of which the eyes are placed. 
In some cases, however, the animals are eyeless.” It is not my 
purpose to enter into their physiological structure, as the paper is 
only intended to describe certain forms which are found in the mud 
bank. Cephalic molluscs inhabit marine and fresh-water; they 
live on land, or, they may be amphibious. ‘This description brings 
us to another division which is confined to those molluscs which 
dwell on land or inhabit fresh-water, and is termed Gasteropoda, 
and here we are introduced to two orders named Pectinibranchiata 
and Pulmonobranchiata, according to their breathing organs. ‘The 
aperture of many of the univalve shells is closed by a curious 
appendage, which is termed the operculum, and which is attached 
to the foot of the creature by a strong muscle. The shells in the 
bank belong exclusively to the Gasteropoda, and among the first 
order Pectinibranchiata, there are a very few which belong to the 
first family of the Noritidee. (I only found four). There is only 
one British genus in this family, Neritinia, and only one species, JV. 
fluviatilis, and I have mounted three specimens, together with the 
operculum, which is peculiar in having on its under side a projec- 
tion, which seems to keep the operculum in a proper position. 
The shell is prettily marked with purple bands, and you may 
notice the broad character of the inner lip of this shell. This 
mollusc inhabits rivers and lakes. The female deposits her capsules 
with from forty-five to sixty eggs upon the shells of neighbouring 
