36 THE CHIGAGO ACADEMY OF SCIENCES. 
IX. ANATOMY OF THE,SHELL. 
For the benefit of those who may npt be familiar with the 
different parts of the shell, but who may desire such a knowl- 
edge to aid them in understanding the relation of the terms 
used in this report to the objects, the following brief discussion 
of the external anatomy is presented:* 
The pelecypods (or lamellibranchiates) are characterized 
(so far as the area under consideration is concerned) by pro- 
counced bilateral symmetry. Two mantle-lobes form the sides 
of the mollusk, and secrete the two flattened, subconical shells. 
These two mantle-lobes are prolonged into tubular siphons. 
The two valves are connected dorsally by a firm ligament, and 
the dorsal portion of the shell is specialized into teeth and 
sockets. Two large adductor muscles close the shell; these 
muscles are plainly indicated on the interior of the shell by the 
scars which theyleave. The exterior of the shell is always cov- 
ered with a thick epidermis. 
In a specimen of Unio luteolus (Plate I.) we will notice the 
following characters: The anterior end (A) short and rounded, 
the posterior end (P) long and quadrate, the ventral margin (V) 
sharp, and the dorsal margin (D) rounded; the ligament (1), 
which holds the valves together; the lines of growth (1g), which 
mark resting stages in the growth of the shell; the lunule (lu), 
a portion of the hinge just above the teeth, and the umbo (u), 
(plural umbones), the nucleus (protoconch), or first part of the 
shell; this will be seen to be wrinkled and corrugated. The 
whole shell viewed from above (middle figure in plate) shows 
an elongated outline, with the ligament (1), lunule (lu), and um- 
bones (u) in the center; the latter are frequently called the 
beaks. 
- On the interior of the shell we notice the following charac- 
ters (lower figure): The long, external ligament which binds 
the valves tightly together, just above the lateral teeth (It); this 
ligament acts as a spring to force the valves apart, the opposite 
from the muscles which pull them together. It is very brittle 
in the dried shell, but in the living animal is tough and firm. 
Beneath the ligament the two lateral teeth (lt) are seen, two 
long, ridge-like processes; in the right valve there is but one 
lateral tooth, which fits in between the two in the left valve. In 
front of these are placed the cardinal teeth (ct), two in number, 
*In preparing this topic the writer has made frequent use of Tryon’s Structural and Sys- 
tematic Conchology, Vol. I., to which he would acknowledge his indebtedness. 
