390 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
A number of elementary anatomical facts are here very briefly mentioned in 
order to make tbe explanation of sections illustrating tbe general anatomy intelligible 
to those not already well acquainted with it. They are also of use in explaining a 
number of scattered observations of my own. 
THE SHELL. 
The outer covering or shell of lamellibranchs is made of an organic base, im- 
pregnated by lime, which is taken from the surrounding water. Osborn (No. 14) and 
others have shown that the mantle lining the shell valves on their inner faces secretes 
a soft, gummy substance, which soon becomes tough and is finally completely filled 
with spicules of carbonate of lime. In the oyster, examined by Osborn, no prismatic 
or mother-of-pearl layer is formed inside this, as is the case in many other forms. The 
varying proportions of lime make a great difference in the resisting power of the 
shell. In the shell of Mytilus edulis , the common mussel, there is but little lime, 
compared with the tough, horny basis, and as a result the shell is strong and unyielding. 
The shell of Venus mercenaria , the “little- neck clam” or “quahog,” as it is often called 
on the Atlantic coast, is, on the other hand, made almost entirely of lime. Though 
the shell is thick and heavy it breaks into many fragments, like a piece of porcelain, 
when struck a severe blow. The thickness of the shell of many lamellibranchs 
depends greatly upon the amount of lime held in solution by the water in which it 
exists. 
It is characteristic of this class of the Mollusca that the shell is made of two 
independent halves, called valves, which are joined to each other by a ligament. This 
is tough and rubber-like, and between it and the concavity of the shell is the hinge, 
where the valves, in touching each other, form a fulcrum. In many cases one valve 
is hollowed out at this point and the other has a corresponding projection, or per- 
haps many of these, each fitting into a hollow. These relations may be seen in Fig. 
96, PI. xciv, where Ig is the ligament and lig the hinge. The adductor muscles (aa and 
pa), on contracting and closing the shell, cause, through the action of the fulcrum at 
the hinge, a stretching of the ligament. When these muscles are again relaxed the 
ligament contracts automatically, as would a piece of rubber, and opens the valves 
at the opposite extremity. 
In the oyster there is no distinct hinge, but the ligament is made of two parts, a 
central, thick, and elastic portion (Fig. 97, PI. xciv, lg), and above and below this a 
slight ridge. The shell projects slightly at these points and may help in functioning 
as a hinge. The valves of the shell are generally equal to one another in size and 
shape. In the oyster the left valve is the larger one; it is much heavier and is suffi- 
ciently hollowed out to contain the whole of the soft parts (Fig. 97, PI. xciv), while the 
right valve is smaller and almost flat. The animal is attached by the former. 
The outer surfaces of the shell are generally marked by concentric lines of growth, 
and along its edges may be found the horu-like cuticle, secreted by the mantle edge. 
This cuticular covering of the outer surface of the shell is often thin and may be lost, 
except at the edges. In the case of Solenomya it is very thick, covering tbe whole 
shell from sight and extending some distance below its ventral edge. The umbo, a 
rounded prominence on the dorsal side of many shells, is well marked in Venus (Fig. 
96, PI. xciv). 
