XAIADGEOGRAPHV OF MISSOURI 29 



was only able to secure two individuals and these were too immature 

 and small to be assigned to any definite Species although thev 

 were so identified as edentulus by recognized students. As indicated 

 in the accompanying Key to the Mussel Faunae these eccentricities 

 of distribution are due to the very different faunal and ecologic 

 conditions. Vet Mr. Bryant Walker, that thorough student of 

 Xaiadgcography, comments: — "There are some very interesting 

 problems connected with the distribution of Missouri Xaiades 

 that should be worked out. The poverty of the fauna of the Mis- 

 souri Valley, as compared with that of either the Upper Mississippi, 

 or of the rivers that flow south through the Arkansas, is verv 

 curious. ... I have never had sufficient data to attempt to even 

 guess at the solution of it." 



As to the depauperated to extinct faunae of the South-East 

 Lowlands and of the immediate waters of the Loess-Alluvial 

 Flood-Plains for the Missouri and Mississippi Rivers the writer 

 agrees with Dr. Paul Bartsch of the Division of Mollusks, U. S. 

 National Museum, and leader of the part}' for the U. S. Pearl Mussel 

 Investigation of the Mississippi River during the summer of 190S. 

 Dr. Bartsch writes : ". . . . we found no Unios between the mouth 

 of the Missouri River and that of the Ohio in the Mississippi. 

 This, I believe, is altogether due to the enormous amount of mud 

 emptied by the Missouri into the Mississippi, making it impossible 

 for the forms to exist there. ... I have reported on the Missouri 

 River as 'The Great Faunal Barrier.'" 



Concerning the distinctive characteristics of the Ozark Fauna 

 Dr. A. E. Ortmann remarks: — "The Ozark region apparently is a 

 continuation oj the Cumberland Plateau in the fauna of its rivers so 

 that there will be geographic and faunistic relations with the 

 Tennessee-Cumberland System." Mr. L. S. Frierson also makes chis 

 comment: — "The appearance of Truncilhe, Pleurobemae and other 

 forms, so intimately resembling those of East Tennessee, in the 

 mountain streams of Missouri and Arkansas is an interesting and 

 remarkable fact illustrating the power of environmental factors." 1 



The writer is in the position to verify the observations of 

 Ortmann, 2 Clark and Wilson 3 and other field investigators, who have 



'An unpublished paper, read before a Washington D. C. Society of 

 Scientists. 



"A. E. Ortmann, Proc. Am. Phil. Soc. LII. Xo. 210. May— Aug., 1913. 



3 H. Walton Clark, and C. B. Wilson. U. S. Bu. Fish. Doc. Xo. 781, 

 PP- 1-23. 



