THE OTSTEE. 35 



has handled an oyster-shell must have observed that 

 it seemed as if composed of successive layers or 

 plates overlapping eacTi other. These are technically 

 termed ' shoots,' and each of them marks a year's 

 gi'ovrth ; so that, by counting them, we can determine 

 at a glance the year when the creature came into the 

 world. Up to the time of its maturity, the shoots are 

 regular and successive ; but after that time they become 

 irregular, and are piled one over the other, so that the 

 shell becomes more and more thickened and bulky. 

 Judging from the great thickness to which some oyster- 

 shells have attained, this moUusk is capable, if left to 

 its natural changes unmolested, of attaining a great age." 

 Indeed, fossil oysters have been seen, of which each 

 shell was nine inches thick, whence they may be con- 

 cluded to have been more than 100 years old. 



For the most part the offspring remains near the 

 mother, which accounts for the large oyster banks or 

 beds which are found in almost all the seas of the 

 temperate and torrid zones, and which in some places 

 have been known to attain such magnitude as to cause 

 ships to be wrecked upon them. The lower stratum is 

 necessarily lifeless, being pressed upon by the upper 

 one, so that the oysters beneath are unable to open 

 themselves, and are consequently deprived of food. 



The immense propagation of the oyster may be under- 

 stood from the fossil oyster bed near Reading, in 

 Berkshire. These fossils have the entire shape, figure, 

 and are of the same substance as our recent oyster- 

 shells, and yet must have lain there from time im- 

 memorial. This bed occupies about six acres, forming 



