Xo. 522] 



THE I ANADIAN OYSTER 



351 



The right, upper, valve remains flatter and smoother, the 

 left, lower or attached valve, more deeply hollowed, 

 rougher, with more conspicuous concentric furrows, and 

 often with blue tipped projecting processes. 



Note on the Asymmetry of the Shell— In the free-swimming larva 



genesis, which is to say that the difference^ in the two valves was first 

 developed after some remote hut free and symmetrical ancestors of 



In living spat the mantle can be often seen protruding 

 beyond the edge of the shell, but in preserved specimens 

 it is retracted, sometimes close up to the gills and body, 

 so that the soft parts of the animal may occupy only a 



