104 MARINE SHELLS. [CHAP. XXVI. 



Mississippi and its tributaries, and, after escaping by 

 one of the many mouths of the great river, had 

 drifted one hundred and fifty miles eastward to this 

 spot. The fact of their long immersion in salt water 

 was sometimes proved by a dense coat of encrusting 

 barnacles, the only marine shells we could find here, 

 for the mollusks proper to this part of the bay are 

 such as belong to fresh or brackish water of the 

 genera Cyrena, Gnathodon, and Neritina. Just before 

 our visit, a north wind had been blowing and driving 

 back the sea water for some days, and the bay was 

 so freshened by the Alabama river pouring in at this 

 season a full stream, that I could detect no brackish 

 taste in the water. It is, in fact, so sweet here, that 

 ships often resort to the spot to take in water. Yet 

 there is a regular tide rising three feet every six 

 hours, and, when the wind blows from the south, the 

 waters are raised six or seven feet. 



After walking over a large expanse of ripple- 

 marked sands, we came to banks of .rnud, inhabited 

 by the bivalve shell called Gnathodon, some of 

 which we dug up alive from a depth of about two 

 inches from the surface. This part of the Bay of 

 Mobile is now the most northern locality of this re 

 markable brackish- water genus, but dead shells of 

 the same species are traced many miles inland, form 

 ing banks three or four feet thick. They are called 

 clams here in popular language, and, being thick and 

 strong, afford a good material for road-making. From 

 the same mud bank we dug out a species of Cyrena&amp;gt; 

 the only accompanying shell. In some places not 

 far off, a Neritina is also .met with. As a geologist, 



