46 THE SHELL-FISH OF THE COAST. 
dry during low-water. One or more forms grow 
on the roots of trees, such as the mangrove, on 
which they remain exposed for hours at a time 
above water. The great thickness of the shell in 
many individuals indicates a long life-period for 
the animal, which has been estimated by some, but 
probably without sufficient basis, to be as much as 
a hundred years. The different layers of the shell 
indicate distinct periods of rest in its develop- 
ment, but at what regular (if regular) intervals 
these periods follow one another is still an open 
question. In the east-coast oysters, whose most 
extensive head-quarters appear to be Chesapeake 
Bay, the shell rarely attains a greater length than 
one foot or 15 inches; but foreign and fossil species 
are known which far exceed these dimensions. 
Thus, a species from the Middle Tertiary deposits 
_of Europe (Ostrea crassissima) measures nearly or 
fully two feet in length; the Ostrea Titan, from a 
somewhat later deposit in California, measures six 
inches through the thickness of the two valves, 
while a recent species (O. Talienwanensis), from the 
Bay of Taichou, Japan, grows to a length of even 
three feet. Careful investigation has shown that 
the American oyster will grow to a length of 
nearly four inches in about two years, and it is 
conjectured from this that in some four years after 
its escape from the egg the animal is approximately 
adult and marketable. 
Oysters appear to thrive best in estuarine coves 
and inlets where the bottom is not liable to shift 
to any great extent, and where the depth of water 
