=Cases of Singular Habitat.=--Mollusca sometimes accustom themselves to living in very strange localities, besides the extremes of heat and cold mentioned above (pp. 23–24). In the year 1852, when some large waterpipes in the City Road, near St. Luke’s Hospital, were being taken up for repairs, they were found to be inhabited in considerable numbers by _Neritina fluviatilis_ and a species of _Limnaea_.[110] _Dreissensia polymorpha_ has been found in a similar situation in Oxford Street, and also in Hamburg, and has even been known to block the pipes and cisterns of private houses. In an engine cistern at Burnley, 60 feet above the canal from which the water was pumped into the cistern, were found the following species: _Sphaerium corneum_, _S. lacustre_; _Valvata piscinalis_, _Bithynia tentaculata_; _Limnaea peregra_, very like _Succinea_ in form and texture; _Planorbis albus_, _P. corneus_, _P. nitidus_, _P. glaber_, and thousands of _P. dilatatus_, much larger than the forms in the canal below, a fact probably due to the equable temperature of the water in the cistern all the year round.[111] In certain parts of southern Algeria the fresh-water genera _Melania_ and _Melanopsis_ inhabit abundantly waters so surcharged with salt that the marine _Cardium edule_ has actually become extinct from excess of brine. The common _Mytilus edulis_ is sometimes found within the branchial chamber and attached to the abdomen of crabs (_Carcinus maenas_), which are obliged to carry about a burden of which they are powerless to rid themselves (see p. 78). A variety of the common _Limnaea peregra_ lives in the hot water of some of the geysers of Iceland, and has accordingly been named _geisericola_.