ORTMANN: FAMILIES AND GENERA OF NAJADES. 261 
and water-tubes well developed, septa rather distant in the male. 
In the female, only the outer gill is marsupial, with crowded septa, 
while in the inner gill the septa are like those of the male. 
When gravid, the water-tubes (ovisacs) of the outer gill swell mod- 
erately, and assume a lanceolate shape, and consequently the placente 
have this shape. The edge of this gill does not distend. The eggs and 
placente are pink or red, and the placente preserve their shape, and 
are discharged whole. This discharge has been actually observed 
in two cases (July 25, 1910). 
Glochidia of the usual shape, rather small, semioval, without hooks.!8 
Color of soft parts very characteristic. All specimens seen had a 
peculiar, pale orange ground-color. The foot, margins of the mantle, 
and adductors were darker, often deep orange. The abdominal sac 
is whitish, the palpi and gills pale grayish brown, the gills with more 
or less of the orange hue. The color may be more or less intense, 
but some shade of orange always prevails. When gravid, the red 
color of the placente contained in the gills blends with the orange of 
the latter, producing a very peculiar lilac tint. 
Plethobasus cooperianus (Lea). 
Only two specimens with soft parts have been observed, the one 
from the Ohio River in Beaver Co., Pennsylvania, the other from the 
Ohio at Parkersburg, Wood Co., West Virginia. Both proved to be 
females. 
The anatomy of this species is practically identical with that of 
P. esopus agreeing in all particulars, chiefly also in the peculiar color 
of the soft parts. This color is so characteristic that it alone suggested 
to me the relationship of cooperianus and @sopus, which was confirmed 
by the subsequent anatomical investigation. No gravid females have 
been observed, and nothing is known about the color of the placentae 
and the shape of the glochidia. 
Genus PLEUROBEMA Rafinesque. 1820. 
Simpson, 1900), p. 745 (amended). 
Shell rounded, subquadrate, oval, or somewhat elongate, but then 
oblique, with the beaks placed anteriorly. Beaks generally somewhat 
13 The glochidia were observed only in one case, in one of the discharging females, 
and apparently were discharged prematurely (a phenomenon observed by others). 
They were young and incompletely formed (soft), but their shape could be made out. 
The size was about 0.18 mm., but it was impossible to make exact measurements. 
