932 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [2} 
upper edge, comprising anteriorly the hinge which unites the shells, is 
extended as two somewhat conical prominences called umbos or beaks. 
The beaks in the Portuguese oyster are unequal in the two valves; the 
left one is always the larger. They are each marked by a groove, vary- 
ing in size, in which an elastic ligament, for opening and shutting the 
valves, is firmly fastened. This groove is also frequently marked with 
ridges and furrows, of greater or less width, which follow its direction 
and indicate the successive lines of growth. 
The edges of the valves in the Portuguese oyster present very con- 
siderable variations; sometimes that of the larger valve is undulated,y 
striated, or denticulated. If the oyster is an old one, the smaller valve 
partakes of the modifications undergone by the larger one, so that it often 
happens that the shells are marked by corresponding indentations. 
Their interior surface is smooth, almost always white, and often pearly 
toward the center; a little back of and above the center, however, there 
exists an oval depression. This is the point of attachment of the cen- 
tral muscle which connects the valves together. In studying the strue- 
ture of the shell of the Portuguese oyster, it is sufficient to saw or 
break the shell in two, in order to become convinced that its structure 
is foliaceous, that is, formed of successive layers. There are two reasons . 
why the specific gravity of this shell is not high. The first is, that 
rather broad spaces often inter- 
vene between the lamin of 
which the shell is formed; the 
second is, that the lamin 
themselves are to a large ex- 
tent composed of a white, po- 
rous substance whose specific 
gravity is exceedingly low. In 
the Portuguese oyster the lami- 
ne, which are irregularly dis- 
tinct, are piled upon one an- 
other like partitions. This NX $4/////, 
laminated structure arises nec- i °& z Sin" Ht stanly Me 
essarily from the organization PZ 
of the animal, and it is known 
that the interspaces contain an : 
acrid water, resulting from a Fig. 1.—The Portuguese oyster. 
depuratory secretion. This phenomenon is, moreover, common among 
the mollusks. 
Let us now carefully open the Portuguese oyster. In the first place 
we must break the solid, dark-brown ligament which, like a hinge, 
attaches the two valves together. But even now the valves do not sep- 
arate. A sharp instrument must be inserted between them, so as to 
cut the cylindrical muscle connecting them near the center.* The ani- 
[* This is not the way oysters are opened in America. The process is reversed ; the 
muscle is first cut with a knife when the hinge is broken apart.—ED. ] 
