37 



localities have been assigned that identify some of the spe- 

 cies of the Tennessee system of drainage with certain forms 

 found in the Alabama system. A careful examination of these 

 supposed cases of identity of species' in the two systems 

 of drainage has not as yet elicited any confirmatory evi- 

 dence. It seems, indeed, very probable, that not a single 

 instance of supposed identity will be verified. In expla- 

 nation, it may be remarked that collectors are not always 

 careful to keep apart from each Bother their unidentified 

 specimens of species from various localities, arid it is not 

 an uncommon circumstance for specimens to pass into the 

 hands of the descriptive naturalist with local references of 

 an unreliable character. This, together with the vague 

 and very indefinite mention of the State or Continent for 

 the locality of a species, is surely the source of continual 

 trouble to one who seeks to reconcile his shells with the 

 literature relating to them. 



While the preliminary sheets of this paper were in pro- 

 gress, the writer was urgently solicited to embody in it 

 such facts in synonymy as might be thought useful. In 

 reply to these solicitations it may be urged that there is 

 not yet at hand, and may not be for many years to come, 

 a sufficient amount of material to enable the most careful 

 student to do justice to the subject, and not at the same 

 time do injustice to those writers who have done most to 

 define species. 



A few of the difficulties of synonomy may be presented 

 in a manner which will be appreciated by those persons 

 who have given the subject slight attention. Take, for in- 

 stance, some common and well known species of Trypa- 

 nostoma, found in the Tennessee river. On tracing it from 

 point to point along the stream it will be found to vary in 

 several particulars, which are obvious at a glance. If only 

 the extreme forms are known, these would appear to the 

 observer to differ so much from each other as to justify 

 him in regarding them as distinct species an opinion he 

 is compelled to abandon when further investigation has 

 brought to light the intermediate forms. There may be, 



