16 



feed upon them; but some species, I should think. 

 were thick enough to defy the masticating powers of 

 that animal. 



1 have observed, that when the waters first fall in 

 the summer, and the bars are suddenly exposed, the 

 Naiades can be procured in greater abundance than 

 later in the season, when the waters have been low 

 for a month or more. Indeed it was only in the early 

 subsidence of the freshets, that I was able to procure 

 some rare and highly interesting species. 



The geographical distribution of the various spe- 

 cies of fresh water shells has been little attended to. 

 In order to attract the attention of naturalists to this 

 t^ubject, I will make a few observations on the facts 

 connected with it, which have been furnished by my 

 recent journey through Alabama. The sources of 

 those rivers which ])our their collected waters into 

 the bay of Mobile, are divided from the tributaries ol' 

 the Tennessee river by a range of mountains of no 

 great elevation, and which are composed of carboni- 

 ferous limestone. The head waters of the Black 

 Warrior are but a few miles distant from the sources 

 of other streams flowing into the Tennessee, yet the 

 general character of the shells is different, a few 

 only being common to the streams, running in oppo- 

 site courses, whilst in the Black Warrior, as fur 



