THE OPEN SEA BEACH 291 



cause lies in the difference of sea temperature in 

 the two regions. On the west coast there is a 

 very gradual slope of the sea bottom for a long 

 distance from the land and the shallow water is 

 winter cooled until its temperature is lowered sev- 

 eral degrees below that of the keys and the south- 

 eastern coast where the shores are bathed by the 

 tepid waters of the Gulf Stream. This powerful 

 current, of mighty volume and majestic flow, is 

 unmodified by Florida winters. Even the shoals 

 and shore water cools but little, hence the marine 

 life is strictly tropical. 



A considerable number of marine mollusks which 

 inhabit the Atlantic coast of the southeastern 

 States are also found in the Gulf of Mexico, but 

 they do not extend their range to the extreme 

 lower part of Florida. The water of the sea, as I 

 have shown, is considerably warmer along the 

 Gulf Stream than it is farther northward and as 

 these are temperate and warm temperate forms 

 they do not find this almost tepid water congenial. 

 For a long time I could not understand this 

 peculiar distribution, nor how these Atlantic coast 

 mollusks could have found their way into the 

 Gulf. Geologists assert that during late Tertiary 



