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TROPICAL NATUREhave evidence of an important change in the distribution of aspeciesof mollusc since the banks were formed.On the St. John's river, Florida, are enormous heapslargely composed of two freshwater shells, Ampullaria depressaandPaludina multilineata, which cover acres of ground,and are often six or eight feet thick. Professor Wyman,who explored these heaps, remarks : "It seems incredible toone who searches the waters of the St. John's and its lakes atthe present time, that the two small species of shells abovementioned could have been obtained in such vast quantitiesas are seen brought together in these mounds, unless at thetimes of their formation the shells existed more abundantlythan now, or the collection of them extended through verylong periods of time. When it is borne in mind that theshell heaps afford the only suitable surface for dwellings,being most commonly built in swamps, or on lands liable tobe annually overflowed by the rise of the river, they appearto be necessarily the result of the labours of a few living ona limited area at one time. At present it would be a verydifficult matter to bring together in a single day enough oflthese shells for the daily meals of an ordinary family."On the Lower Mississippi, at Grand Lake, are shell banksof great extent which are now fifteen miles inland ;while Nottand Gliddon describe similar banks on the Alabama River fiftymiles inland, and they believe that Mobile Bay must haveextended so far at the time the shells were collected. Thesebeds are often covered with vegetable mould from one to twofeet thick, and on this grow large forest trees. Equally indicativeof long occupation and great antiquity is the enormousshell mound at San Pablo, on the bay of San Francisco, whichis nearly a mile long and half a mile wide, and more thantwenty feet thick. Numerous Indian skeletons and mummieshave been found in it, showing that it had been subsequentlyused as a place of burial. Some mounds in Florida havegrowing on them enormous live oaks from thirteen to twentysixfeet in circumference at five feet from the ground, someof which are estimated to be about 600 years old, indicatingthe minimum age possible for the heaps, but not necessarilyapproaching to their real age.1 Fifth Annual Report of Pedbody Museum, p. 22.