164 Annals of the Carnegie Museum. 



lake tributaries in Ohio, and living under similar conditions. It 

 seems that no serious mistake will be made if this is done, for there 

 is no evidence to show that post-glacially the shells have become 

 extremely modified. Moreover the shells from the Maumee are 

 comparatively few in number. 



If I am asked why on the other hand only specimens from streams 

 nearest the divide are considered, it will be clear from the literature 

 quoted, and substantiated by the opinion of excellent systematists 

 that in the lower stretches of the Upper Ohio Drainage "big stream" 

 conditions have so modified and are so modifying the Naiades, that it 

 would be extremely difficult, if possible, to use the same species in a 

 number of cases. In others, however, a paucity of material, where 

 there is reason to believe the species is not excessively varying, has 

 led to the utilization of material from larger streams. 



V'll. Results. 



The values obtained from these calculations are found on Table IV. 

 For the convenience of the writer as well as of the reader the compari- 

 sons derived were then tabulated as shown in Table III from which 

 all written conclusions are derived. Table II gives the maxima, 

 minima, and mean of all dimensions for each species of shell from the 

 two environments, affording closer insight into Table III. The 

 actual measurements of the shell from which the material in Tables 

 II and III is derived, while really belonging here, are placed at the 

 end of the paper for the sake of convenience. 



VHI. Conclusions. 



The outstanding facts to be gathered from Tables II and III are: 



1. All shells, with exception of Anodontoidcs ferussacianus obtain a 

 greater degree of inflation in Lake Erie than in the Ohio. It follows 

 from this and other evidence, which I have presented, that shells are 

 longer in the Upper Ohio. 



2. The height of a shell [DVD), appears to be indifi"erently influ- 

 enced by either environment according to these measurements, 

 although it could be stated that most species tend to be "higher" 

 in the Upper Ohio and the tributaries of Lake Erie. 



3. The evidence shows that the great majority of species tend to 

 have a greater posterior development (PD), in Lake Erie, and greater 

 anterior development (AD) in the LIpper Ohio, etc. 



