624 ORTMANN— NAYADES OF 



the intergrades were not known. But the discovery of the latter — 

 and 'this was one of the problems to which I directed my attention — 

 has necessitated the cancellation of a great number of these nominal 

 species. Nevertheless, the fauna still remains remarkably rich, 

 which surely in a large part is due to the comparatively old age of 

 this river system, to the diversity of its character, and also to certain 

 changes of the drainage which have taken place in the geological 

 past. 



In the following pages, the correct names of the various forms 

 are given, conforming to the systematic arrangement published by 

 the writer (Ortmann, 1910, 19126), and conforming with the rules 

 of priority. It should be remarked, that of practically all Tennessee- 

 forms the anatomy has been investigated, but has not yet been pub- 

 lished of all of them: the description of the rest will appear in due 

 time. The two great papers of Simpson (1900 and 1914) are taken 

 as a basis, and the quotations are from the last paper, so that it can 

 he easily seen, where changes in nomenclature have been introduced. 

 Also the names used by Lewis, those used by Pilsbry and Rhoads, by 

 myself and Goodrich are given, in order to facilitate comparison 

 ■with our list. The synonyms are all quoted unless they have been 

 recognized and accepted as such by Simpson; but other references 

 have been largely omitted, for the reason that they generally are 

 found in Simpson's paper. 



No full descriptions of the forms are given, but frequently the 

 chief characters are briefly indicated. 



The extralimital distribution has not been given in detail. Here 

 and there it has been referred to, but only in especially interesting 

 cases. In this respect, much work remains to be done, and in many 

 North American Nayades the exact boundaries of the distribution 

 have not yet been exactly located. 



The material, upon which the present paper is founded, has been 

 deposited in the Carnegie Museum of Pittsburgh, and comes chiefly 

 from the collections made by myself ; a very small part has come 

 from other sources. The Carnegie Museum is in possession of the 

 old collections of Hartman, Holland, and Juny, and a more recent 

 collection has been bought from H. H. Smith. In addition, the 

 museum is indebted to Messrs. Frierson and Walker for occasional 



