280 MEMOIRS OF THE CARNEGIE MUSEUM 
the system of the North American Najades, and to put it upon a more secure 
foundation. A preliminary account of this work has also been published (Ortmann, 
1910). 
The most prominent advance in our ideas as to the systematic arrangement 
of the fresh-water mussels is marked by the publication of Simpson’s paper on the 
Najades (Simpson, 1900). We cannot but admire this work, for the general prin- 
ciples of classification are correctly recognized by Simpson, and the soundness of 
his views has been amply confirmed by our own investigations. Yet Simpson 
suffered under two serious disadvantages: he did not possess enough material repre- 
senting the soft parts to settle all systematic questions, and did not make any 
microscopical investigation of the anatomical structures. This latter shortcoming 
has been in part remedied by Sterki (Sterki, 1903). In the present monograph 
additional observations will be furnished, and with regard to the Pennsylvanian 
species, it may be said that the soft parts of all of them, with very few exceptions, 
have been at hand, and they are now sufficiently well known to enable us to assign 
each species to its proper place in the system. Of course in the case of the species 
and genera not found in Pennsylvania, this reference remains to be made; but 
after the main points of view have been indicated, this will be a comparatively 
easy task. The fact, however, should be emphasized that there is now no excuse 
whatever for collectors of the Najades preserving only the hard parts, and throwing 
away the soft parts. The soft parts form an integral portion of the animal, and 
should be preserved and examined together with the hard parts, and this is most 
imperative in the case of those genera, of which our present knowledge is more or 
less defective. 
The greater portion of the material at hand, forming part of the collections 
belonging to the Section of Invertebrate Zodlogy in the Carnegie Museum, was 
secured by the writer in person. The art of collecting mussels has to be learned, 
because, while it is easy enough to pick them up, it can only be done after a thorough 
knowledge of the proper places and seasons has been acquired. The Najades 
inhabiting our rivers, creeks, ponds, and lakes, can only be obtained alive by going 
after them into the water, but as soon as their whereabouts is discovered, it is 
easy enough to get them. Their discovery cannot be accomplished under all con- 
ditions. Part of the season our creeks and rivers have too much water, or the 
water is too muddy to locate the mussels. This is the case chiefly in early spring, 
when the snow melts, and in the first half of summer, the time of the copious 
summer rains. 
In Pennsylvania the best season is generally from August to October, when 
