ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 327 
Glochidia generally of medium size and normally of simple shape, semicircular 
or semielliptical, without spines; yet in certain forms peculiar glochidia are found 
(See p. 301). Placentz: more or less well developed, but often atrophied, so that 
the glochidia appear free. The discharge of the glochidia takes place in an un- 
usual manner, through the edge of the marsupium. 
The essential characters of this group (marsupium and edge of mantle) un- 
doubtedly are connected with the prolonged breeding season (all these forms are 
bradytictic), and with the unusual manner of discharging the glochidia. The soft 
parts are built according to a special plan, which is adapted to this peculiar dis- 
charge, and facilitates the proper aération of the glochidia during the long breeding 
season, but this latter adaptation is different from that seen in the Anodonta-group 
Serving the same purpose. 
Shape of the shell very variable, round and heavy, or more or less elongated 
and lighter. Color of epidermis dull or bright. Sculpture of beak generally more 
or less double-looped, more rarely concentric, often obsolete. Hinge mostly well 
developed, with strong teeth, rarely somewhat reduced. Male and female shells 
more or less differentiated, sometimes only very slightly, but in other cases the 
differences between the shells of the two sexes are very striking. 
Remarks: The essential structure of the marsupium of this group has been 
entirely overlooked by Simpson. He, indeed, brings these forms together in his 
system, but groups them according to characters, which are rather unimportant. 
In addition, his generic divisions are not at all satisfactory, and need complete 
revision, as we shall see below. Sterki also has recognized the mutual affinity 
of these forms, but on account of the similarity of the glochidia to those of the 
genera of the first group (Quadrula, etc.), he brings them too near to the latter. 
The structure of the marsupium in this group is again so unique, that it alone 
suffices to sharply separate the group from all others. 
I do not think there should be any doubt or hesitation in recognizing these 
groups as natural. The differences are such and so uniform, that we must regard 
them as fundamental. They concern in the first instance the sexual apparatus, 
and the development and specialization of the structures, which serve the physio- 
logical process of propagation, we are compelled to take as the surest indication 
of a phylogenetic progress within the group. One of the most important char- 
acters of the fresh-water shells, called Najades, is found in the way they take care 
of their young, not discharging the eggs directly into the water, but keeping them 
for a time in the gills; and it is only natural that this peculiar feature, after it 
had once developed, is further improved, complicated, and specialized. 
