38 THE CHICAGO ACADEMY OF SCIENCES. 
fulcrum. The ligament (I.) acts upon the short arms of the lever 
(the umbones, II., III.) as a spring, tending to keep the valves 
apart, or gaping. The adductor muscle acts upon the long arms 
of the lever (X., XI.) and by its contraction tends to keep the 
valves together. It will readily be seen that the mollusk is not 
at rest when the valves are tightly closed, for this requires some 
exertion on the part of the animal to pull them together and 
overcome the spring of the ligament. The period of rest is 
when the shells are slightly gaping. 
The study of the structure of the shell is of great importance 
in understanding its formation and relation to the animal. If 
the shell of a Unio be broken so as to show a fractured edge, and 
studied with a powerful hand lens, the following structure will be 
seen (Fig. 3): An outer or epidermal layer, composed of almost 
Fie. 3. 
Diagram of a section of the shell of Unio TUBERCULATUs, Barnes, showing 
the shell layers. a, epidermis; b, columnar layer; c, prismatic or inner 
layer. (Original.) 
black tissue (a); a layer of columnar tissue (b) set at right 
angles to the epidermal layer; anda prismatic layer (c) forming 
the inner surface of the shell, and composed of thin plates of 
membrane and carbonate of lime placed alternately. The func- 
tion of the epidermis is to protect the more calcareous portion 
from erosion by the carbon dioxide (CO,) in the water. The 
effect of this acid is seen on the umbones of the shells where 
the epidermis is thin or'deficient. In most of the Unios the 
epidermis is green, olive green or black in color, and frequently 
rayed. It is formed by the cells on the edge of the mantle. 
Breaks which occur in the edges of the shell are easily supplied 
with the epidermal covering, but when they occur in the body 
of the shell, as at the upper third, no epidermal matter is 
deposited, but only nacreous matter, showing that it is the mar- 
gin of the mantle which forms the epidermis. 
