XXXVlll 



species been foimd numerous within a hundred 

 miles of our coasts ; nor do they approach the more 

 northern Lititudes of the Middle and Western 

 States, very few species being- found so far north as 

 the Ohio River. 



The Mississippi River also, seems to have formed, 

 from the junction of the Ohio until its mouth, an 

 insurmountable barrier to the geographical disjjer- 

 sion of these shells. 



Thus, we find the district of our country, which 

 they inhabit in such profuse numbers of species and 

 individuals, to be really of somewhat limited extent, 

 and may give its boundaries as follows : — JVorthy the 

 Tennessee River and tributaries. The Cumberland 

 Mountains prevent the dispersion of the species 

 of this river to the northward until its course is 

 directed into Alabama. Here the character of its 

 species (which we shall again allude to farther on) 

 changes, and they become gradually less numerous 

 and of greater geographical dispersion, as the river 

 runs towards the west, ^ast, the mountain range 

 of the Blue Ridge, running southwestwardly into 

 the interior of Northern Georgia. Thence, the 

 Chattahoochee River and tributaries, to within about 

 a hundred miles of the Gulf. South, the species 

 are restrained from spreading by the influence ot 

 the Gulf of Mexico. West, the Alabama, Cahawba 

 and Black Warrior Rivers and their tributaries, 

 those of the latter reaching almost to Florence, on 

 the Tennessee River, which may represent the 

 northwestern point of our boundary'". 



These limits are necessarily imperfect, but never- 

 theless include at least three-fourths of our species 

 within an area of three hundred miles extent, 

 either north and south, or east and west. 



Of course, where the rivers alone form the 

 boundaries, many of their species have spread into 

 the adjacent streams ; but in East Tennessee, south- 

 western Virginia, western North Carolina and north- 



