362 Mr. Osler on burrowing 
cata ; it crawled about almost without intermission for the 
greater part of two days, and at length fixed itself in a cre- 
vice by the side of the ligament. During the five subsequent 
days it was perfectly quiescent : it perished at the expiration 
of that time, apparently destroyed by the putrefaction of the 
ligament. I have taken many hundreds of the animals from 
their holes, and kept them in confinement till they died, but 
never saw one of them attempt to change its place. 
It is evident that the Saxicava does not bore like the Pholas, 
by a rotatory motion, for the hole is not quite round, and 
its smallest diameter is not equal to the depth of the Saxicava, 
from the hinge to the ventral margin. The animal there- 
fore has not room to turn itself ; and its attachment by a 
short byssus imposes an additional restraint upon it : indeed, 
an examination of its muscles is sufficient to show that they 
cannot effect a rotatory motion. The only boring motion 
which can be supposed to be given to the shell is that effected 
by the action of the muscles of the foot ; that, in fact, by 
which a Bivalve buries itself in the sand ; but this, although 
so well adapted for travelling over a smooth surface, or for 
burrowing into a substance penetrable by the foot, would be 
ineffectual here, because the animal being already in contact 
with the rock, the foot cannot be projected to form a fixed 
point beyond the shell. Even were it otherwise, the texture 
of the shell is so soft, that it could make no impression upon 
the stone without being itself acted on ; and the effect of this 
would be permanent, because superficial injuries of the shell 
are never repaired. But nothing of this kind is met with. 
I have even found a Saxicava fixed between two others, which 
was so compressed that it was quite flat, and little more than 
