65 



considered by the best English conchologists as a true native, 

 being found in many different localities where the chalk or 

 oolite prevailed, though not hitherto met with in the imme- 

 diate neighbourhood of Bath. The -ttw/a*, or lid, which gives 

 this species its name, was said not to be an opercle, properly 

 so called, having no organic connection with the animal, but 

 being merely a calcareous covering, secreted by the mantle 

 at the approach of winter, in order to close the aperture of 

 the shell against cold and wet, and cast off on the return of 

 spring : beneath this calcareous lid might be found two or 

 more other membranous coverings, easily separable, occupy- 

 ing lower and lower positions in the cavity of the mouth 

 as the animal retreated further into the interior. This snail, 

 like some other helices, retains its vitality for a long time if 

 kept shut up in a dry place, and may often be revived after 

 lying dormant for a whole year or more. 



The Dreissena polymorpha, said to have been imported 

 with timber from the Baltic in 1824, but like the Helix 

 pomatia, probably indigenous, was mentioned as occur- 

 ring in great quantities in the locks of the canal at Bath, 

 where it had been seen on one occasion, when the water had 

 been drawn off for the purpose of repairs, so thickly coating 

 the sides of the lock as scarcely to leave any unoccupied 

 space. 



The President, after making remarks on some species of 

 mollusca which, though outwardly very similar in the form of 

 the shell, differ much in the structure and habits of the ani- 

 mal — as also on other species, in which the shell itself varies 

 greatly, according to age and the peculiar conditions of the 

 locality in which it is found — cautioned geologists as to how 

 they established new species in their palaeontological re- 

 searches, in which they had nothing but the shell to guide 

 them, and the charactei-s of the same often ill-defined from 

 the ages elapsed since being tenanted by its inhabitant 



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