436 
TROPICAL NATURE 
VIII 
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.” 1 
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 Peabody Museum, p. 22. 
