PHOLADIDJE. PIDDOCK. 3 



more than an inch deep. Its shell being about two 

 inches long, this arrangement left about an inch and 

 three-quarters exposed. After a short time the animal 

 attached its foot to the bottom of the hole, and com- 

 menced swaying itself from side to side, until the hole 

 was sufficiently deep to allow it to proceed in the 

 following manner. It inflated itself with water appa- 

 rently to its fullest extent, raising its shell upwards 

 from the hole; then, holding by its muscular foot, it 

 drew its shell gradually downwards. This would have 

 produced a perpendicular and very inefficient action, 

 but for a wise provision of nature. The edges of the 

 valves are not joined close together, but are connected 

 by a membrane ; and, instead of being joined at the 

 hinge, like ordinary bivalves, they possess an extra 

 plate attached to each valve of the shell, which is 

 necessary for the following part of the operation. In 

 the action of boring, this mollusk, having expanded 

 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, gradu- 

 ally opening the lower anterior edges and closing the 

 upper, thus boring both upwards and downwards. 

 The spines (points) on the shells are placed in rows, 

 like the teeth of a saw ; those toward the lower part 

 being sharp and pointed, whilst those above, being 

 useless, are not renewed. So far for the operation of 

 boring; but how to account for the holes fitting the 

 shape of the animal inhabiting them ? To this I fear- 

 lessly answer, that this is only the case when the 

 Pholas is found in the rock which it entered when 

 small. This mollusk evidently bores merely to protect 

 its fragile shell, and not from any love of boring ; and 



B 2 



