R'^DER.i XJFE-mSTORY OF THE OYSTER. 333 



is due to crowding, and also to the sedimentation or silting of sand 

 or earth between such individuals as grow closely together on the bot- 

 tom. In the struggle for existence the animal is impelled to grow up- 

 wards from these causes in order to reach its food and the water nec- 

 essary for respiration. Consequently the shelly deposit is laid down by 

 the mantle mainly at the free ends of the upwardly directed valves, so that 

 the latter grow only in length and not in width, thus giving rise to the 

 extremely elongated type often met with. Such a form sometimes pre- 

 vails over a whole bed, and the valves are often relatively very thin in 

 consequence of the rapid growth which has been made in only one 

 direction. In this form the animal or soft parts are also much elongated 

 antero-posteriorly, so as to be quite different in shape as compared ^vith 

 the soft parts of an individual, such as that of which the shells are rep- 

 resented on Plates LXXVII and LXXVIII. 



