ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 325 
and there is heavy tissue at the edge capable of being stretched out. This causes 
a rounded or truncated edge in the charged marsupium. This peculiar structure 
of the marsupial gill is developed only in the gravid female, and is absent in the 
sterile female. 
These characters are apparently connected with the prolonged breeding season, 
and the peculiar secondary water-tubes serve for the aération of the embryos in 
the marsupium. The stretching of the edge of the gill is easily understood. 
Other characters are the following: marsupial gills always extremely swollen 
and marsupium always formed by the outer gills alone in their whole length; supra- 
anal opening well separated from the anal; the separating connection of the margins 
of the mantle well developed, sometimes short, but normally somewhat longer, 
and even quite long, thus leaving only a short slit for the supra-anal; branchial 
opening well defined anteriorly; no papilla or other special structures on the edge 
of the mantle in front of the branchial opening; inner lamina of inner gills free 
from the abdominal sac, more rarely more or less connected with it; glochidia 
very remarkable, and furnishing a very important character of this group. They 
are generally very large, subtriangular, in rare instances almost semicircular in 
shape, and are more or less pointed in the middle of the ventral margin, where 
they possess a peculiar spine or hook (this spine is undeveloped in very young 
glochidia). The glochidia are sometimes imbedded in well developed placentze 
or placentula. Sometimes the placentx are poorly developed, or the glochidia 
appear completely isolated. The discharge is by the natural channels and the anal 
opening, and these forms are bradytictic. 
The essential characters of this group are found in the shape and structure 
of the marsupium, which represent adaptations to the extreme swelling of the 
latter, and to the long breeding season, for all forms belonging here are bradytictic. 
The formation of lateral water-canals in the marsupium undoubtedly is a device 
to provide proper aération for the glochidia enclosed so long a time in the mar- 
supium. Whether the special hooks of the glochidia are connected with a dif- 
ferentiation of function from that in the Quadrula-group is yet unknown. 
The shell of the forms belonging to this group is never very heavy and solid, 
and never distinctly rounded, but it is more or less elongate, rather thin, often 
with distinctly colored epidermis (banded and rayed). The sculpture of the beak 
is of two types: either more or less distinctly double-looped, or concentric; in the 
latter case often very heavy. The hinge-teeth have a distinct tendency to 
become obsolete. They are present in some forms, but are then often of peculiar 
shape, and in others they exhibit all stages of reduction to complete disappearance. 
