MOLLUSKS OF FLORIDA 
177 
identical with West Indian ones does not necessarily imply a 
recent introduction. Some mollusks, at any rate, seem to have 
preserved their specific characters unchanged through several 
geological periods. On the other hand, although there cannot 
be the slightest doubt that a certain number of species intro¬ 
duced by human agency thrive in other localities besides their 
native homes, I am not convinced that mollusks spread across 
any wide expanse of sea by other accidental transport. With 
Mr. Bryant Walker * * * § I prefer to attribute the tropical land 
mollusks of Florida largely to a former land connection 
between the then island of Florida and a larger southern land- 
mass. I cannot, however, agree with Mr. Walker’s view that 
this event took place in comparatively recent times. Dr. 
Simpson urges that the Floridian area must have been joined 
to the greater Antilles by way of the Bahamas in Eocene 
times. Nevertheless, he does not derive the tropical species 
of Florida from the southern invasion which must have taken 
place at that time. He favours a recent colonisation by acci¬ 
dental transport. The rich fauna, of the Bahama islands seems 
to him entirely derived from the greater Antilles in that 
manner.f 
There are certain geological grounds for the supposition 
that an ancient Archaean land-mass trending north-eastward 
from the northern end of the Andes once existed, and that 
traces of it are still recognisable in Guatemala, Cuba and 
Haiti.J Much of this early land may still have stood above 
sea-level in early, and perhaps middle, Tertiary times, form¬ 
ing a centre from which the North American continent de¬ 
rived part of its present fauna. 
Dr. Ortmann § demonstrated in a very convincing manner 
that the fresh-water crayfish belonging to the genus Cambarus 
originated in Mexico, spreading from this centre of dispersal 
into the United States at the beginning of the Tertiary Era. 
The centres for the more advanced forms of the sub-genus 
Cambarus, and for the sub-genera Faxonius and Bartonius, 
* Walker, Bryant, “ Origin of American Mollusca,” p. 56. 
t Simpson, 0. J., “ Land and Freshwater Mollusks of West Indian 
Region,” pp. 447—448. 
X Frazer, T., “History of Caribbean Islands,” p. 398. 
§ Ortmann, A. E., “ Affinities of Cambarus,” pp. 124—125.” 
L.A. 
N 
