(ancient) ROMAN BATHS IN BATH. 137 



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, ' Hydrebia ventrosa^^ came to 

 be in a spot so far removed from 'brackish water,' their well known 

 proper habitat. My 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 effect- 

 ually 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 speak of the contents of the mud bank I have described. 



The MoLLUSCA form a sub-kingdom of the animal world, 

 divided into — i. Acephalous, without a head; 2. Cephalic, having 

 a head. The former division have Bivalve, the latter Univalve 

 shells ; of the latter only I have to speak, because I have dis- 

 covered no bivalve shells in the deposit of the bath. The Cepha- 

 lic 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 this paper is only intended 

 to describe certain forms which are found in the mud bank. 

 Cephalic molluscs inhabit both 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 that dwell 

 on land or inhabit fresh-water, and is termed Gasteropoda ; 

 here we are introduced to two orders, named Pedinibranchiata and 

 Pulmonobranchiata^ according to their breathing organs. The 

 aperture of many of the univalve shells is closed by a curious 

 appendage, termed the operculum, which is attached to the 

 foot of the creature by a strong muscle. The shells in this stratum 

 belong exclusively to the Gasteropoda; and, among the first order, 

 Pectinibrajichiata^ there are a very few which belong to the first 

 family of the Neritides. (I have only found four.) There is only 

 one British genus in this family, Neritina^ and only one species, 

 N, fluviatilis. I have mounted three specimens, together with 



