27 





Figure 3. 

 with the hinge end downwards and the free edges of the 

 valves upwards the animals are in an excellent position 

 to feed while the outside vertical surfaces of the valves 

 are well adapted to afford places of attachment for the 

 ''spat." The latter however seems to attach itself in the 

 greatest abundance to the old oysters fixed at the sur- 

 face of the bank. If one removes oysters from these 

 beds they are found to adhere together in clusters as 

 shown in Pigs. 4 and 5. Carefully observing the char- 



Figure 4. Figure 5. 



acters presented by these masses, it is soon noticed that 

 they consist of a series of generations which have grown 

 one upon the other. In Fig. 4 four generations may 

 easily be niade out, as indicated by the letters a, &, c, d: 

 in ascending order the oldest generation a, is represent- 

 ed by dead shells, which are buried in a black, sandy 

 material, which has acquired solidity from the large 

 quantity of imbedded dead shells which it contains. 

 Attached to the upper edges of the dead valves of a fol- 

 lows generation ^, which consists in part of living and 

 in part of the shells of dead animals ; on the preceding 

 the living uppermost generation c is fixed, which affords 

 attachment to a generation of young d, though where 

 there are open cavities the lower generations may bear 



