MOLLUSKS OF FLORIDA 177 



identical with West Indian ones does not necessarily imply at, 

 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. $ 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, C. J., " Land and Freshwater Mollusks of West Indiap 

 Region," pp. 447448. 



t Frazer, T., "History of Caribbean Islands," p. 398. 



Ortmann, A. E., " Affinities of Cambarus," pp. 124125." 



L.A. N 



