306 MEMOIRS OF THE CARNEGIE MUSEUM 
Considering the anatomical structure of the gills, and the fact, that the embryos 
are contained in the water-tubes, we should expect, a priori, that the discharge 
would take place by the natural channels. This would be by the way of the 
suprabranchial canals into the cloacal chamber, and thence outward through the 
anal opening of the mantle. And indeed, I have collected evidence for this natural 
discharge in a number of cases, which furnish positive proof of it. They are the 
following: In cutting up specimens of Quadrula subrotunda, on June 22 and 24, 
1909, I repeatedly observed placentze coming out of the anal opening. Of 
course, this might be due to rough handling, but I have never seen them coming 
out at any other place, unless I forced them out by so strong a pressure, that 
the walls of the gills burst open. On June 24, 1909, I collected a good number 
of specimens of Quadrula subrotunda and coccinea, which I took home alive, 
and kept over night in a bucket with water. The next morning I saw quite 
a number of placentz floating in the water and was able to distinguish easily those 
belonging to the first species, which are subeylindrical and pink, from those belong- 
ing to the second, which are flattened (leaf-shaped or lanceolate) and white. 
On June 23, 1910, I took home a number of gravid Quadrula coccinea, and 
watched the discharge of the placentz. They distinctly came out of the anal 
opening. On June 24, I opened one of these specimens, and saw the placentie 
slowly moving out from the water-tubes through the suprabranchial canal to the 
anal siphon. While I was holding this specimen in my hand (with the shell opened 
and the left half of the mantle thrown back), four placentze moved out. The 
coming out was not by a jerk or a squirt, but was slow and gradual, as if carried 
by a steady current of water. A second specimen, opened shortly after this, 
exhibited the same phenomenon. On July 25, 1910, two specimens of Plewrobema 
wsopus were found in the act of discharging placentz. One was seen in the water, 
with a number of pink placentz lying behind the posterior end of the shell, which 
were greedily devoured by a number of minnows. The other was taken home, and 
the discharge was observed at leisure in a basin of water. The pink placentz came 
slowly out of the anal opening, often in pairs, and sometimes, when the shell was 
squirting water out of the anal opening, a number of placentze were thrown out. 
Among my alcoholic material, there are several individuals, which were pre- 
served in the act of discharging glochidia in the natural way. The best specimen 
is an Anodontoides ferussacianus, collected May 14, 1908, which has the posterior 
half of either marsupium empty (discharged), while the anterior half is yet full. 
Here I observe, that the suprabranchial canals of the marsupial gills are crowded 
with loose glochidia back to the cloacal cavity, and a mass of glochidia is located 
