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Dreissena, one species, D. polymorpha, Pall. This shell 

 attaches itself firmly to the stones or woodwork on which it 

 lives, by means of a byssus, in exactly the same way as Mytilus 

 edulis, L., the common mussel of the seashore. There is a 

 theory that it is not an indigenous species, but was introduced 

 by timber ships from the Baltic ; but it is more reasonable to 

 suppose that it has become distributed by natural means, as 

 it is common throughout the whole of the north of Europe, 

 including Russia. It is fairly common throughout the 

 district in running water and ponds, and in London has even 

 made its way from the New River, and been found in profusion 

 in iron waterpipes taken up in Oxford Street. Neritina ; 

 N. fitiviatilis, L., the only species, is of frequent occurrence, 

 and is found commonly throughout the Thames as far as 

 Hammersmith and Barnes ; also in the Sussex Ouse, and in 

 the Avon at Christchurch. Pahidina, two species, one P. 

 vivipara, L., common throughout ; the other, P. contecta, 

 Millet., rarely met with. The first named is the shell so often 

 seen for sale, to put in aquaria with gold fish. Bythinia, 

 two species, both common. Valvata, two species, both 

 very common. Planorbis : of the twelve species in this genus, 

 only one is absent, PI. dilatatus, Gould. This shell occurs in 

 a few places in Lancashire, and is generally considered an 

 introduced species imported to Manchester in bales of cotton. 

 Physa, two species, P. fontinalis L., and P. hypnorum, L., 

 both widely distributed. There is a third, Physa acuta, 

 but its only claim to rank as British, is that for many years it 

 has inhabited a tank in, I think, the Victoria Regia House at 

 Kew. Where it originally came from is not known with 

 certainty; but it is a common European species, also found in 

 the West Indies, at Cuba, St. Thomas, etc. Limncea : of 

 eight, the only one absent is Limncea invohUa, Thompson, 

 a shell only found in a little mountain lake near Killarney. 

 There are probably no shells whose outward form is so 

 directly the result of external conditions as those of the genus 

 Limncea. I have brought a good many examples of L. peregra, 

 Miill., the most variable one, to illustrate this. As a matter of 

 fact, almost every pond has its own variety. In a pond near 

 Tooting a sinistral variety of this shells occurs. This form is 

 exceedingly rare. I know no other locality for it in England ; 



