50 THE CHICAGO ACADEMY OF SCIENCES. 
ular dyssus organ (bg) situated in the left side of the mantle 
between it and the shell.* 
In this stage there are no auditory organs, gills or velum 
and the digestive cavity is a simple pouch with thick walls and 
a single large opening. The embryo of Anodonta reaches this 
stage in a few days after fertilization and remains in this stage 
without change until the following spring, when the parent dis- 
charges them through the cloacal siphon into the water. Here 
the embryos attach themselves by the byssus to the fins or gills 
of small fishes, close the valve of the shell onto the body of the 
fish by driving the hooks of the ventral surface of the valves 
into it, The fish covers the larva with a growth of epithelial 
cells and the embryo becomes encysted. Here it develops 
gills, esophagus, stomach, intestine, renal organs and heart, and 
finally escapes from the cyst, falls to the bottom and completes 
its growth. + 
Ovules filling the entire outer gill of the female; ovisacs 
not separated by a sulcus. (Simpson.) 
Genus ANODONTA (Bruguiére £m) Lamarck, 1799. 
Shell: Generally thin, oval or oblong, inflated, without sculp- 
ture, anterior end evenly rounded, posterior dorsal region ele- 
vated or developed into a wing which meets the posterior end at 
a greater or less angle; epidermis generally smooth and nearly 
or quite destitute of rays; beaks concentrically sculptured; hinge 
line regularly curved, edentulous, wot incurved in front of the beaks; 
nacre generally dull; muscle scars not impressed. (Simpson.) 
Animal: Much like that of Unio, but the anal opening is 
always destitute of papille; outer gill of female when gravid 
enormously thickened and pad-like. (Simpson.) 
Distribution: North America, Europe, Asia and North 
Africa, north of the Desert of Sahara. 
KEY TO SPECIES OF ANODONTA. 
A. Shell large. 
1. Breadth generally moderate, compared with length; color 
dark green or blackish, more or less distinctly rayed; 
umbonal sculpture: Very COarsSe <5 ieee > sia» «i+ os as ere ines grandis 
*The byssus is sometimes retained after the mollusk is well on its way to maturity and 
such an organ has been found in Unio ligamentinus Lam., eight inches in length, the shell 
being 27 mill. long. The mollusk was found attached toa stone by the byssus. See Sterki, 
The Nautilus. Vol. V., p. 73. 
+See Brooks, pp. 330-332. 
