142 



;mil foimd attaclied to rocks, tlu' sliclls are slmrt and stuhliv, whorls well thick- 

 ened and witli iiierassate ai)erture. The same siiells obtained from pools wiiere 

 the water does not How at all. and where vegetation flourishes in great abundance, 

 are elongate, thinner in texture, thinner about tlie ajierture, have the lines of 

 growth far ai)art and well marked. These are the points on which the supposed 

 distinct species iiave been based, but are thus seen to lie but a reilex of the con- 

 ditions of environment. 



The (T<ini(>l)(i!<rx present the same facts, but since they are often found at- 

 tached to the faces of vertical rocks, from which they do not migrate very far, 

 there is a very characteristic modification of the aperture which results, evidently, 

 from the efli'ects of gravitation. The final paper will present many facts which 

 tend to this explanation of the difierent forms of aperture, which, as is well 

 known, determines the real form of the shell. 



A few important observations on the animals themselves have been made, but 

 these regard chiefly minor anatomical details and possess little general interest. 

 Enough has been learned, however, to determine that several species, at least, 

 have been based upon the sexes. This difference is seen in the general outline of 

 the female shell, which has always characteristically well-rounded whorls, a con- 

 ditifMi itself a result of the positions of certain organs within the l)ody of the 

 animal. 



Several of the forms found at the Falls of tlie Ohio are of wide geogi'aphic 

 distribution. These limits have been determined and a study made of the shells 

 as expressive of differences in the conditions of the several stations. 



The most abundant species of Anciilom found at the Falls ranges to the rivers 

 of middle Alabama, and occurs overall the region of east Tennessee, in the larger 

 streams. Coincident with this wide distribution there is a great diversity of form, 

 and thus there has arisen a rather large syni)nymy, which it is the puri)osc of this 

 studv to establish. Not less than twenty times has Anciilom pni-roKd been de- 

 scribed by as many (UfFerent conchologisls who j»ublished from scanty material 

 and with the understanding that every stream had its own forms. My own studies 

 in this connection are based upon extensive collecting over all this wide region and 

 on very large (piantities of the shells of the several species. The material from 

 the Falls of the Ohio alone, which has passed under observation, comprises some- 

 thing over two bushels of shells. In tiie ([uantity one who does not recognize, as 

 Lea did not recognize, the modern notions j)ertaining to species and the extent to 

 which they respond to geographic factors, might erect forty species with as great 

 luojiriety as one. 



