524 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, 1912b), 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 
be 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 
