356 
Mr. Osler on burrowing 
itself firmly. When it is thus fixed, the lateral muscles, 
acting in an oblique direction, will raise the posterior end of 
the shell, and press its armed extremity forward and down- 
ward : or, if one of them should contract more strongly than 
the other, it will bring down the corresponding side of the 
shell, which will be restored to its erect position by the action 
of the opposite muscle. 
The Pholas has two methods of boring. In the first, it 
fixes itself by the foot, and raises itself almost perpendicu- 
larly, thus pressing the operative part of the shell upon the 
substance to which it adheres : it now proceeds to execute a 
succession of partial rotatory motions, effected by the alter- 
nate contraction of the lateral muscles, employing one valve 
only, by turning on its side, and immediately regaining the 
erect position. I have observed that this method is almost 
exclusively employed by the very young animals ; and it cer- 
tainly is particularly adapted for penetrating in a direction 
nearly perpendicular ; so that they may be completely buried 
in the shortest possible time. It may be observed, that the 
posterior extremities of the valves are much less produced in 
the very minute Pholades than they afterwards become ; and 
thus the time required to complete a habitation is still farther 
diminished. 
But when the Pholades have exceeded two, or, at the 
utmost, three lines in length, 1 have never observed them to 
work in the manner I have described : the altered figure of 
the shell, and the increased weight of that part of the animal 
behind the hinge, would prevent it from raising itself so per- 
pendicularly as at first, independent of the narrow space 
which it now occupies. In the motions required to enlarge 
