ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 295 
identical with that of the male in all sterile individuals. There is no change in 
this matter in the gravid females of the genera Unio, Pleurobema, and Quadrula. 
The result is that the edge of the marsupial gill does not participate in the swelling 
of the gill. The greatest swelling in these genera occurs near the base or in the 
middle of the gill, while it decreases toward the edge, which always remains rather 
sharp, and this sharp edge of the marsupial gill is a very important macroscopical 
character in the species of these genera. A microscopical examination shows that 
there is no difference of the structure of the edge from that of the sterile female 
(Compare Pl. LXXXVII, figs. 18, 14, 16; Pl. LXX XVIII, fig. 1). 
In our other Unionide it is different, and we distinguish two types of develop- 
ment, though in all cases the differentiation concerns only the edge of the outer, 
or marsupial gill. 
The first type is found in the Anodonta-group (in those forms, which have 
the water-tubes divided into three tubes; as described above). It has already been 
said that in these forms the marsupium swells to an extreme degree, and this 
immense swelling also affects the edge. The two lamine of the gill draw apart 
at the edge, and this strain would cause a rupture at this point, if it were not for 
the fact that there is here an extraordinary thickness of tissue, which is capable 
of stretching, and thus keeps the edge of the gill closed (See Pl. LX XXVIII, figs. 
2b, 3b, 5, 10). There are no filaments, or chitinous rods, or ostia visible in this 
elastic tissue. 
This transverse stretching of the edge of the gill takes place in various degrees, 
and is well observed only in marsupia, which are fully distended. Even in fully 
charged marsupia it is often absent in certain parts, chiefly so toward the anterior 
and posterior ends of the edge of the gill, which then remains simple, but appears 
as a kind of ridge or cord upon the swollen marsupium. In the middle of the gill, 
however, this stretching takes place regularly, and is sometimes accompanied by a 
slight bulging out of the edge, beyond the original margin of the gill (PI. LXXXVIII, 
figs. 36, 10). In other cases such bulging is hardly noticeable, and the edge of 
the gill does not appear rounded off, but rather truncated (Pl. LXXXVIII, figs. 
2b, 5). The peculiarities of this structure may be easily seen with the naked eye 
or with an ordinary magnifying glass, and the most striking effect is that the edge 
of the marsupium in these forms does not appear sharp, as in the Unio-group, but 
blunt, rownded off, or truncated. The single water-tubes (or ovisacs) are not dis- 
tinetly marked off externally along the edge of the marsupium. 
The other type of the specialization occurring on the edge of the marsupium 
is found in the Lampsilis-group, to which belong the following genera and species 
of Simpson’s “Synopsis,” females of which I have investigated: 
