366 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Continuing the contrast between flat and Portuguese oysters the following differ- 
ences are noteworthy : 
FLAT OYSTER. 
Shell grows laterally at the expense of thickness ; 
is usually light, friable, very large in comparison to 
size of animal; its margin is undulated by a dozen or 
more flattened projecting ridges. The lateral method 
of growth is marked by a circling fringe of straw- 
colored cuticle, dentelle. The saucer-like valve is the 
more robust at the free margin and indicates out- 
wardly, by slight asperities, the concentric rings of 
growth. These are sometimes outlined by a slight 
violet tint. The upper valve is usually quite flat, 
showing from within a broad band of translucent cu- 
ticle often an inch in width. Outwardly this valve 
is shingled with irregular overlapping bands of horn- 
colored cuticle. The nacre is frequently pearly, of a 
pinkish cast. The hinge is slight, allowing the oys- 
ter to be opened hinge wise. The breadth of ligament 
is suggestive of Pecten, a likeness often made still 
moro striking by the flat, angular processes thrown 
out by the shell on either side of the hinge. The shell 
moreover is circular and ridged, like Pecten; its mus- 
cular impression, too, is central and usually colorless. 
A degree of movement might therefore be suspected, 
though the adductor muscle does not appear rela- 
tively large. 
The mantle is delicate and broad-margined, con- 
tracts vastly, forming irregular plaits ; its sensory 
margin is wide, generally pale pinkish or brownish 
in color ; papillae abundant, small, and needle-like ; 
posterior flaps of mantle wide and prominent. The 
transparency of the mantle permits the viscera, liver, 
intestine, and rectum to be generally outlined. The 
crystalline style is dense and prominent. 
In habits, the edulis, as before noted, gradually 
separates from the object to which it had attached. 
In muddy localities it is enabled to survive by open- 
ing and shutting its flattened scallop-like valves, to 
retain a horizontal position. When out of water the 
quick snapping of valves and forcible ejection of 
water are noteworthy. With gradual shifting of bot- 
tom the oysters are enabled to alter their position 
considerably, tending, it is said, to congregate in 
banks. In all of the oyster ponds examined during 
the summer the density of the water was found to be 
extreme, practically that of the sea, of ameansp.gr. 
of about 1.026, and in range from 1.020 to 1.028. In 
relation to the question of spawning it is remarkable 
that at the two centers of seed production, Auray and 
Arcachon, the water densities have been found lower 
(1.022 to 1.024) than at other localities examined. It 
would naturally appear therefore that, aside from the 
question of silt deposit, probability for spat-catching 
seems to decrease as the water increases in saltness. 
PORTUGUESE OYSTER. 
Shell shaped roughly like a human foot, but 
distinctly pointed at the heel. The external 
ridges become at the margin deeply accented, 
forming notches grotesquely toe-like. The 
valves thicken rapidly, their plane of growth 
becomes tilted and undulate, and their limy 
growth extends to the free margin of the shells, 
with cuticle lacking. The deep valve is trench- 
like, with usually a recess under the hinge, and 
with free edges appearing to arch over. Its 
depth measured externally is often five times 
that of the opposing valve. Its outward asper- 
ities are prominent and irregular. The upper 
or right valve is depressed, but resembles out- 
wardly the lower one. The nacre is limy and 
irregular, often greenish in color, and often 
darkened in patches where invading mud 
masses have been cemented in. Hinge ends of 
shell heavy and pointed, that of lower valve 
produced backward, spur-like. Hinge liga- 
ment, located in deep notch, is stout and de- 
fends the oyster from being opened hingewise. 
The muscular impression, usually purple, is 
small, oblong antero-posteriorly, its muscle 
attaching the shell near the median of the pos- 
terior margin. The oyster is therefore opened 
by a knife thrust at the side. 
The oyster is sunken in its deep shell. Its 
mantle is narrow and opaque, outlined with a 
broad, jet-black, sensory band. The sense pa- 
pill® are finger-like in shape, large and long. 
Posterior flaps of mantle not marked. 
The Portuguese oyster is naturally a clus- 
tered form, occurring in shallow water of a 
mean normaPtemperature of 65 to 70° F. The 
weight of its cluster keeps it firmly rooted in 
its native muddy sand. All of its conditions 
of living appear to be those of the O. virgini- 
ana in South Carolina ( vide U. S. F. C. Bull., 
1890, p. 336). The clusters are often of great size, 
composed of a hundred or more individuals. 
The oyster separated from its cluster and grown 
on a sandy bottom improves materially in flesh 
and shell. It is a littoral form, though not 
occurring in waters notably freshened, the 
densities over the oyster beds at the mouth of 
the Tagus (middle of September) ranging from 
1.023 to 1.0255. 
