TROPICAL NATURE 



have evidence of an important change in the distribution of a 

 species of mollusc since the banks were formed. 



On the St. John's river, Florida, are enormous heaps 

 largely composed of two freshwater shells, Ampullaria de- 

 pressa andPaludina multilineata, which cover acres of ground, 

 and are often six or eight feet thick. Professor Wyman, 

 who explored these heaps, remarks : "It seems incredible to 

 one who searches the waters of the St. John's and its lakes at 

 the present time, that the two small species of shells above 

 mentioned could have been obtained in such vast quantities 

 as are seen brought together in these mounds, unless at the 

 times of their formation the shells existed more abundantly 

 than now, or the collection of them extended through very 

 long periods of time. When it is borne in mind that the 

 shell heaps afford the only suitable surface for dwellings, 

 being most commonly built in swamps, or on lands liable to 

 be annually overflowed by the rise of the river, they appear 

 to be necessarily the result of the labours of a few living on 

 a limited area at one time. At present it would be a very 

 difficult matter to bring together in a single day enough of 

 these shells for the daily meals of an ordinary family." l 



On the Lower Mississippi, at Grand Lake, are shell banks 

 of great extent which are now fifteen miles inland ; while Nott 

 and Gliddon describe similar banks on the Alabama River fifty 

 miles inland, and they believe that Mobile Bay must have 

 extended so far at the time the shells were collected. These 

 beds are often covered with vegetable mould from one to two 

 feet thick, and on this grow large forest trees. Equally indica- 

 tive of long occupation and great antiquity is the enormous 

 shell mound at San Pablo, on the bay of San Francisco, which 

 is nearly a mile long and half a mile wide, and more than 

 twenty feet thick. Numerous Indian skeletons and mummies 

 have been found in it, showing that it had been subsequently 

 used as a place of burial. Some mounds in Florida have 

 growing on them enormous live oaks from thirteen to twenty- 

 six feet in circumference at five feet from the ground, some 

 of which are estimated to be about 600 years old, indicating 

 the minimum age possible for the heaps, but not necessarily 

 approaching to their real age. 



1 Fifth Annual Report of Pedbody Museum, p. 22. 



