ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 331 
The Pennsylvanian species placed under Unio are unquestionably more closely 
allied to each other than to those placed under Pleurobema, but there are other 
extralimital species, which cannot be so easily associated with this genus. Further- 
more, we have several types included in the genus Plewrobema which are rather 
distinct, and which may be entitled to generic rank, unless we prefer to leave all 
these forms united in the old genus Unio. The Pennsylvanian material representing 
the species of this group is scarcely extensive enough to enable us to settle this ques- 
tion. There area multitude of forms of Unio and Pleurobema in the central and 
southern parts of North America, many of which apparently form connecting links. 
Before these have been more closely studied, and before it has been positively ascer- 
tained that they belong in this group, it is futile to attempt a final classification of 
theseforms. Thus I retain the main divisions (Unio and Pleurobema) introduced by 
Simpson, adding to the latter genus the species removed from Quadrula. With 
regard to Q. cooperiana it is absolutely certain that it stands close to another 
species placed by Simpson in Pleurobema, P. esopus, and is still more closely allied 
to P. cicatricosum. With regard to Q. coccinea, pyramidata, and obliqua, this is 
not so evident, but I think that there are some southern types of Plewrobema, 
with which these should be associated. 
I assume that Quadrula having four gills serving as marsupium, represents a 
more primitive stage than the others, where only the outer gills have this func- 
tion and the corresponding structure. It appears, as a matter of course, that the 
differentiation between the gills, the inner serving exclusively the function of breath- 
ing, and the outer exclusively that of propagation, represents a higher stage of 
development than in those cases in which all gills serve both functions; and this is 
further supported by the fact that in certain species of Quadrula (subrotunda, 
kirtlandiana, rubiginosa) we have types, in which the swelling of the marsupium 
in the breeding season is so small, that the water-tubes and their contents remain 
subeylindrical, which undoubtedly is a primitive condition. The fact that the 
most primitive form, Margaritana, has also all four gills marsupial, further speaks 
for our assumption. Yet I cannot suppress the thought that the opposite opinion 
has something in its favor, and the evidence for this is furnished by the fundamental 
differentiation of the edge of the inner and outer gill, as is shown by my description 
of the peculiar furrow along the edge of the inner gill, which is absent in the outer 
gill. As stated above, I cannot surmise what the meaning of this feature is. It 
may be possible that it represents an old, primitive differentiation of the two gills, 
the edge of the outer gill indicating the original restriction of the function of propa- 
gation of this gill. Then, of course, we would have to assume that in Quadrula 
