ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 297 
tion in this, for instance in Obliquaria it is only a small part of the gill immediately 
behind the middle which forms the marsupium. But this is, in my opinion, rather 
unimportant, for in the typical forms of Lampsilis, Obovaria, and Plagiola very 
often the outmost posterior end of the outer gill is not used as marsupium. But 
it is without exception a rule that the marsupial part always projects beyond the 
original edge of the gill (as marked by the filaments); and this is also noticed, 
although in a lesser degree, in the sterile female. When the gills are not charged, 
of course, the protruding, outbulging tissue is much contracted; yet the mass of 
this tissue is so considerable, that it is even then clearly seen, and it keeps the 
lamine of the gill permanently apart at the edge (See Pl. LXX XVIII, figs. 13, 15, 
17, 21), so that the beaded appearance is also preserved. By this character a 
female of these forms may always be recognized even when sterile, and further 
the marsupial edge may generally be distinguished by its color. Often it appears 
simply opaque whitish, in distinction from the water-soaked, transparent tissue 
of the rest of the gill, but in other cases a peculiar dark pigment develops, chiefly 
on the inner side of this tissue in the epithelial layers (See Pl. LX XXVIII, figs. 
15, 17, 18, 19, 20). 
The part of the outer gill of the female which is not used as marsupium has 
the normal structure, that is to say, it corresponds essentially to the structure of 
the edge of the outer gill of the male. In sterile females it simply forms the 
anterior continuation of the marsupial part, and is not marked off, except by the 
lesser elevation of the edge (Pl. LXX XVII, figs. 6, 7, 10). In the gravid female, 
however, the marsupium swells principally at the edge, and it also increases its 
longitudinal (antero-posterior) dimension. Thus it is forced to push past the 
anterior part of the gill, forming a fold with it (See Pl. LXX XVII, fig. 5). This 
fold probably is analogous to the numerous folds seen in the genus Ptychobranchus 
(Pl. LXX XVII, fig. 3), at any rate it serves the same purpose. Sometimes, when 
also the hindmost part of the outer gill does not take part in the formation of the 
marsupium, a similar small fold is also seen posteriorly, but this is rather insignifi- 
cant and variable. At the anterior fold, the marsupium pushes forward on the 
inside of the anterior, non-marsupial part of the gill. 
C. THE OVA AND THE LARV& (GLOCHIDIA).! 
1. Arrangement of the ova and embryos in the marsupium. 
As is well known, the sexual orifices are situated on each side of the abdominal 
sac, and open into the anterior part of the suprabranchial canal of the inner gill. 
‘Wherever I use the words “ova” or “eggs,” as distinguished from ‘‘glochidia,’’ I mean the eggs and their subse- 
quent stages of development, before the glochidial shell is formed. 
