ORTMANN: FAMILIES AND GENERA OF NAJADES. 317 
lies in the outbulging part of the gill, and only the basal ends extend 
very little in between the original gill-lJamine.” 
In young specimens, the marsupium is simple, and forms a smooth, 
compressed (not much swollen) body, marked off from the anterior 
and posterior non-marsupial parts of the gill by irregular folds. In 
larger specimens, however, this marsupial mass begins to warp, and 
finally is folded up into a number of irregular folds. The strongest 
folds are near the anterior end of the marsupium. In none of my 
specimens does the marsupium occupy the whole margin of the gill. 
The placente are quite solid and 
permanent, and possess a_ peculiar 
structure (see Plate XVIII, fig. 11). 
In all specimens at hand glochidia are 
developed, and they appear arranged 
around a central axis (placenta), the 
color of which is white or red. The 
latter color, if present, is restricted to 
this axis, and the glochidia themselves 
are transparent white, and form a 
fringe around the narrow edges of the 
placente. They seem to be connected 
with them by fine threads, possibly 
Fic. 18). Left gills of a large 
gravid female (shell 60 mm. long), 
from Cumberland River, near Ro- 
wena, Russell Co., Ky. (Carn. 
their embryonal threads.“ Since the Mus., No. 61, 4,966.) 
placentze themselves almost touch the 
septa, the glochidia are thus crowded toward the lateral faces of the 
marsupium. Whether this arrangement is already present in the 
eggs, is unknown to me. It is, however, certain that this arrangement 
can only have the purpose of bringing the glochidia as close as pos- 
sible to the wall of the marsupium, in order to give them the best 
chance to be near the current of fresh water going over the mar- 
supium. This is one of the little special devices for the proper 
aération of the glochidia. 
Glochidia of unique shape; they are much longer than high, and 
40 Simpson says that the marsupium occupies the “‘ base”’ of the outer gills. This 
is a very ambiguous expression, but apparently is intended to imply that it is 
situated on the margin of the gills, while the ‘“‘base’’ is non-marsupial. This is a 
very peculiar feature of Dromus, and not met with in any other genus. Only 
Ptychobranchus has the same condition slightly indicated. 
41 The glochidia adhere rather firmly to the placental mass by their threads 
and it is hard to isolate them, except with caustic potash. 
