16 
feed upon them; but some species, I should think 
were thick enough to defy the masticating powers of 
that animal. 
I 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 
subject, 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 pour their collected waters into 
the bay of Mobile, are divided from the tributaries of 
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 opp6- 
site courses, whilst in the Black Warrior, as far 
