SHORES OF THE CLYDE AND FIRTH. 
69 
US turn for a moment to the structure of the creature. The 
strong-ribbed, powerful valves form the source of protection 
for the delicate and beautiful interior ; the pale white 
mantles that surround the corrugated edges guard against 
depredating intruders, and as they open up their parting 
edges the elegant foot is exposed within. To this instru- 
ment, which is less than a fourth of the size of the shell, is 
ascribed the sole power of burrowing. 
Suppose, then, we take the relative proportion of the 
foot to be a fourth of the size of the entire shell ; if we put 
the simple mathematical problem, can the fourth of a whole 
excavate a cavity large enough to hold three times its own 
size? The question seems somewhat ridiculous, and on 
these grounds, to the minds of the writers mentioned, so 
might also the process a^»ove described. But this is not the 
solution. If the cockle is found upon the ebbed beach, 
it is simply there like a stranded vessel, perfectly unable to 
move until the next tide brings it a sufficient quantity of 
water to enable it to do so. Like the urchin, it is quite 
impossible for him to drag his bulky valves below until the 
water completely covers him over, then, lying upon his edge, 
with the beaks parallel with the sand, he sets the foot down 
its whole length into the sand below, and taking in by the 
mantle on the opposite edges above a sufficient quantity of 
water, sends it Avith force enough down the sides of the 
foot to its tip below, which instantly causes an explosion of 
the particles of sand, and these rising like a cloud of dust 
up the outer surface of both sides of the valves, allows the 
whole shell to drop into the cavity as far as the explosion 
has gone. In this manner the process is repeated until the 
creature has gone as far down as it wishes to go, which is 
seldom as deep as three inches. During the months of 
May, June, and July, these shellfish are much sought after 
