



The Nautilus. 



Vol. v. JUNE, 1891. No. 2. 



ON THE MEANS OF DISTRIBUTION OF UNIONID.S: IN THE 

 SOUTHEASTERN UNITED STATES. 



BY CHAS. T. SIMPSOX, WASHINGTON', D. C. 



In the March number of the Nautilus Mr. S. Hart Wright in .some 

 notes on the distribution of Unionidce in the Southeastern United 

 States thinks it remarkable that many of the species of Georgia and 

 the Carolinas should be found in Southern Florida, and that they 

 should pass from one stream to the other. I think Avhen we fully 

 consider all the conditions of environment of these mollusks it will 

 be easy to understand how they have migrated. 



The region is one of excessive rainfall. Over a wide area of 

 territory bordering on the Gulf and the Atlantic it is nowhere less 

 than 45 inches a year, ranging up to 75 inches in the vicinity of 

 Cape Hatteras. North of latitude 30° the most of this moisture 

 falls during the winter and early spring. South of somewhere near 

 that line the regular dry and rainy seasons of the tropics set in, and 

 nearly all the precipitation occurs in June, July and August. Of 

 course these conditions vary somewhat with different seasons and 

 under different circumstances, but as a rule in any part of this area 

 the gi-eater amount of the annual rainfall occurs within a limited 

 period, producing extensive floods. 



It is well known that all the region indicated, or at least a wide 

 belt of it extending along the Atlantic and Gulf coasts, is exceedingly 

 low and flat, consisting for the most part of level, sandy plains, 

 rising as a rule but a few feet above, and alternating with swampy 

 streams. 



