58 FEBRUAHY. 



through the weaker one, as if it had been merely a piece 

 of chalk rock" 



Mr. Eoss, of Ehyl, having a Pholas in his aquarium, 

 prepared a piece of wood, by excavating a shallow cavity, 

 about a quarter of an inch deep, in which he set the 

 animal, whose shell was two inches long. "After a 

 short time the animal attached its foot to the bottom of 

 the hole and commenced swaying itself from side to 

 side, until the hole was of sufficient depth to allow it 

 to proceed in the following manner : It inflated itself 

 with water, apparently to its fullest extent, raising its 

 shell upwards from the hole ; then holding by its mus- 

 cular foot, it drew its shell gradually down. This 

 would have produced a perpendicular and very ineffi- 

 cient action, but for a wise provision of nature. The 

 edges of the valves are not joined close together, but 

 are connected by a membrane (extension of the mantle), 

 and instead of being joined at the hinge (umbo) like 

 ordinary bivalves, they possess an extra plate, attached 

 to each valve of the shell, which is necessary for the 

 following operation : In boring, this mollusk, having 

 dilated itself with water, draws down its shell within 

 the hole, gradually closing the lower anterior edges until 

 they almost touch. It then raises its shell upwards, 

 gradually opening the lower anterior edges, and clos- 

 ing the upper, thus boring both upwards and down- 



