ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 329 
I think that the families and also the three subfamilies of the Unionide 
are geologically rather old, and we cannot say that one descended from the other. 
They probably go back to a common ancestral group, which probably stood some- 
where between the present Margaritanide and Unionine. It is not my object, at 
present, to go into detail in this respect, and to discuss the phylogeny of the Na- 
jades. This will be possible only after a fuller knowledge is at hand of the exotic 
forms, and by studying also the fossil Najades. But I may here incidentally 
mention that I believe that these main groups probably go back to the Cretaceous, 
but the Lampsiline may be a little, but not much, younger. This is also expressed 
in the geographical distribution, the Lampsiline alone being restricted to North 
America (including Central America), while the others are also found in the Old 
World. But I shall leave this question for the present. 
B. THe GENERA. 
1. The Margaritanide consist solely of the genus Margaritana. 
2. The Unionine contain the following genera of Simpson: T'ritogonia, Unio, 
Pleurobema, Quadrula. With regard to Tritogonia, Sterki (Nautilus, 21, 1907, p. 
48) was the first to point out that it possesses the marsupium attributed by Simpson 
to the genus Quadrula, and my own investigations have convinced me that this 
is quite correct, and that Tritogonia is a true Quadrula. The other three genera 
fall according to Simpson into two groups. In Unio and Pleurobema only the 
outer gills serve as the marsupium, while in Quadrula both outer and inner gills 
become charged with ova. 
My investigations have shown that there are indeed a number of species, in 
which all four gills are marsupial, while there are others, in which this is true only 
of the outer gills. This is an absolutely constant character, and I have not seen 
a single instance in any species, where there was any variation in this respect. 
Thus it is evident, that this character, emphasized by Simpson, is important, and 
it is indeed, so far as I can see, the most important one which marks a division 
within this subfamily. Thus we must retain the genus Quadrula in the sense that 
it comprises those species, in which all four gills serve as marsupium. However, 
Simpson placed in the genus Quadrula a number of species, which according to my 
observations have the marsupium formed only by the outer gills. This he did, 
of course, because he did not know the marsupium. As long as we were unable 
to recognize the shape of the marsupium, unless we had gravid females, it was 
impossible to assign the proper place to any species, in which the gravid female 
was unknown. But since I have shown that the sex of any specimen may be 
