OF THE GULF OF MANAAR. 
91 
the left valve is the larger and the thicker and lies under- 
most, and, except when free, immovably cemented for life 
to a rock. In the pearl oyster it is the short ear of both 
valves that is weighted so as to fall downwards conveniently 
for the foot to catch the bottom, while the other prolonged 
ear is lightened presumedly for the same pm-pose of facili- 
tating the falling of the animal foot-downwards. The weight- 
ing of the byssal and foot ear also provides strength in 
the testaceous covering just where force is exerted by the 
animal, for it is from this point that the shell and animal 
is moved and moored. The shell of the young pearl oyster 
is covered with numerous thin flattened spines radiating in 
a series of lines from the umbo, proceeding one above the 
other from the irregularly concentric lines which mark the 
periodicity of the growth of the shell from contour to contour, 
and standing only just free from each other and overlap- 
ping the shell. They are said not to be present in the older 
pearl oyster. I had no means of observing for myself, 
for we came on no fuUgrown pearl oysters. The whole 
shell of the immature pearl oyster, with the exception of the 
umbo and the byssal ear, is very thin and fragile, but in the 
mature oyster the shell thickens and becomes consequently 
heavier and stronger. The results of this arrangement in the 
changed habits and greater protection of the animal will be 
seen hereafter. 
The Interior. 
Contrary to the rule with lamellibranchiate moUusks, the 
pearl oyster, though a mussel, resembles the English oyster 
in being monomyanj or one-muscled, that is, it has but 
one adductor muscle with which to bring its valves together 
at will ; when this muscle is relaxed, the shell opens in- 
voluntarily from the thick elastic cushion formed by the 
middle of the hinge Ugameut being compressed between the 
valves, and acting like a spring cushion to open them. Thus 
