52 BULLETIN OF THE 
with the rear end of the exterior gills, which follow an opposite direc- 
tion (PL III. Fig. vrr.). 
The size, the form, and the structure of the interior gills are precisely 
as in Anodonta. The small conical divisions, or cylindrical cavities, 
which are found between the membranes, open into the distended canal, 
which henceforward we shall term the gill-sac (Pl. III. Fig. viir., 1x). 
An examination of its membranes under the microscope reveals the 
existence of delicate parallel lines running from above downwards; they 
extend also over portions of the gill proper, and are due probably to the 
blood-vessels uniting them. I could not detect between these lines the 
existence of the delicate transverse lines generally found on the gills. 
The convex edges of the gills exhibit delicate protuberances (Pl. III. 
Fig. vi, Vil; 1x, 2)" 
Coming to the generative organs, I have been able to make out the 
ovaries only; they are on the sides of the abdomen, in the rear, and they 
are bounded anteriorly by the liver and the large curves of the intestine ; 
as previously stated, the rear part of the intestine runs between them 
before it comes to the duodenum (Pl. III. Fig. vi. 2; 1x. y). 
Each ovary is composed of a number of small cylindrical sacs or cap- 
sules, pointed at the end and lying in close juxtaposition to each other, 
without, however, being connected; the pointed end, and that only, is 
attached to a strong membrane inside the ovary. 
The ovaries, when they answer this description, are not impregnated ; 
when impregnated, the sacs or capsules increase in bulk, the end is no 
longer sharp but rounded, and they assume the shape of a pear; the 
posterior portion of the abdomen is distended, and becomes thicker, and 
the sacs increase in size and gradually protrude from the abdomen. 
Whether this protrusion is due to the bursting of the membrane of the 
abdomen, or whether the sacs protrude through openings for the purpose, 
has not yet been ascertained. 
By this protrusion, these capsules are introduced into the gill- 
canal, or the gill-sac, but they are still connected with the interior 
membrane of the ovaries by the stalks or filaments at the sharp end; 
the other parts of the capsules are free, or lie close up on the interior 
surface of the membrane of the gills, but not attached to them; but 
few of these capsules penetrate into the gill-sac at one time (Pl. III. 
Fig. 1x. A; x.) 
* In the gills of Anodonta 1 have found the débris of the shells of parasites, and though 
in the gills of Cyclas (Spherium) I have detected masses composed of fine sand or gravel, 
I have failed to discover the presence of any such débris. 
