282 MEMOIRS OF THE CARNEGIE MUSEUM 
I cannot close these introductory remarks without making public acknowledg- 
ment of the encouragement and valuable help furnished to me at all stages of my 
work by Dr. W. J. Holland, the Director of the Carnegie Museum. First of all, 
the study of the Najades forms part of Dr. Holland’s general scheme and his 
policy of studying our local fauna (See Holland, W. J., in the Proceedings of the 
American Association of Museums, Vol. III, 1909, p. 12) and thus the general idea 
of this work, as well as my previous work on the crawfishes (See Memoirs of the 
Carnegie Museum, II, 1906), is largely due to his suggestion. In addition, he 
supplied me with the necessary means for carrying on my collecting, and the chief 
item in such work is the expense. As to this, I can only express my great gratifica- 
tion and sincere thanks for the liberal allotment of funds made to me. Finally, 
Dr. Holland furnished me with all the paraphernalia for the work of the laboratory, 
including everything that belongs to its literary and technical side. It goes without 
saying that he always followed the progress of my studies with the closest attention, 
helped me along in difficult cases, and was ever ready with suggestions and advice. 
The wealth of the subject matter to be treated makes it necessary to divide 
this memoir into parts. Since it is absolutely essential to lay a proper foundation 
for the systematic part, and since the anatomical studies furnish this foundation, 
it is obvious, that the latter should be given first. In the present publication 
therefore I shall take up questions of anatomy and their bearing upon the system 
of the North American Najades. I reserve for a future publication the description 
of the various species and their zo6geography. 
PART I. ANATOMICAL INVESTIGATIONS. 
The anatomy of the European Unionide appears to be rather well known, 
at least after the fashion in which they are treated in the textbooks. (I have con- 
sulted those of Cooke, 1895, and of Taylor, 1894 and 1900.) We have also several 
accounts of the anatomy of American forms (for instance, Brooks, 1890). But 
looking more attentively at these, we find that comparatively few genera and 
species have been studied. This is not astonishing in the case of Europe, where 
only a few types are found. With reference to the American shells, however, it 
is very strange that we possess descriptions, more or less satisfactory, only of 
Unio and Anodonta, and only of eastern species of these genera. Further, we 
shall see that even in these some of the most important parts of the anatomy have 
been neglected. 
With regard to the numerous other genera found in the great interior basin, 
we possess only scanty notes on the gross anatomy. These have been supplied 
