THE NAUTILUS. 63 



by Dr. James Ritchie : ' ' Even our seas have been enriched by 

 strange aliens which have clung through thick and thin to 

 their hosts during the vicissitudes of transportation. An in- 

 teresting recent example is furnished by the appearance of 

 the American Slipper Limpet (Crepidula fornicata) in the 

 Thames estuary. The first sign of its presence there was a 

 dead shell found on the shore at St. Osyth in 1891, although 

 a fisherman had recollection of the 'Crow oyster' extending 

 back some fifteen or twenty years. In 1893 a living example 

 was found amongst oysters from the River Crouch, and there- 

 after records came with ever-increasing frequency, until it 

 was discovered that the Slipper Limpet, from being a rarity, 

 had become a pest. Its numbers on the oyster beds became 

 so troublesome that endeavors were made to eradicate it, a 

 special crushing apparatus being arranged for converting 

 into manure the 'Limpets' dredged from the bottom. About 

 1911 the Blackwater Fisheries alone yielded 35 tons of Slipper 

 Limpets in four weeks; and since then the multiplication of 

 the alien has been even more rapid, for in twelve months in 

 1914-15 upwards of 1000 tons, dredged chiefly from the estu- 

 aries of the Blackwater and the Coin, were crushed and used 

 for manure by the farmers of the district. The precise rela- 

 tionship between the Slipper Limpet and the oyster is un- 

 known, but whether the former be a semi-parasite or only a 

 constant messmate, there seems to be little room for doubt 

 that it was introduced with foreign and probably American 

 oysters brought for relaying in the oyster beds of the Thames 

 estuary. ' ' 



In the same work is the following note bearing on a fresh 

 water mussel : " In 1824 the Linnean Society received the first 

 recorded British specimens of the zebra mussel (Dreissensia* 

 polymorpha), these having been found in abundance at- 

 tached to shells and timber in the Commercial Docks on the 

 Thames. The zebra mussel lives in fresh water in the Danube 

 and the rivers of Russia, and in northern France, Belgium 

 and Germany. It is supposed to have been originally carried 

 to Britain with cargoes of wood from the Volga, and it has 

 actually been seen attached to Baltic timber ere yet the 



