- PHOLADTD^.-— PIDDOCK. 
159 
says, Having procured several of these mollusks in 
pieces of tinaber, I extracted one and placed it loose 
in my aquarium, in the vague hope that it would per- 
forate some sandstone on which I placed it. It pos- 
sessed the powers of locomotion, but made no attempt 
to bore. I then cut a piece of wood from the timber in 
which it had been found, and placed the Pholas in a 
hole a little 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 commenced 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 ap- 
parently to its fullest extent, raising its shell upwards 
from the hole ; then, holding by its muscular foot, it 
drew its shell graduallv 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, gradually opening the 
lower anterior edges and closing the upper, thus boring 
both upwards and downwards. The spines (points) o-n 
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 
