and boring marine animals. 361 
to that of the Bivalves which burrow into sand. The number 
and attachments of its muscles, as well of the adductors as 
those connected with the foot, are exactly the same. The foot 
is directed nearly horizontally forward, and in its state of con- 
traction is very small, but it admits of being distended and much 
elongated : I have repeatedly observed it extended in young 
animals to the length fully equal to that of the shell : when in 
this state, it is seen to contain an opaque white line or tube, 
which passes forward from the part where it joins the body, 
and terminates abruptly on the under surface near its ex- 
tremity. This tube becomes black when the animal has been 
preserved in a saturated solution of muriate of soda. The 
mantle is of a greyish white colour, opaque, and remarkably 
thick. A byssus arises from a longitudinal slit in the pos- 
terior part of the base of the foot ; and passing out with this 
organ through the opening in the mantle, attaches the animal 
to some part of its hole. 
I have had an opportunity of observing the young of 
Saxicava rugosa, before they had begun to bury themselves. 
The activity which they displayed was a perfect contrast to 
the extreme sluggishness of the older animals. They tra- 
velled over the stone by extending and fixing the foot, and 
then bringing the shell forward by the action of its anterior 
and posterior muscles, exactly in the same manner as the 
locomotive Bivalves. In their progress they turned the foot 
in different directions almost at every step, appearing to ex- 
amine the surface of the stone for a suitable spot on which 
to fix themselves. At length they attached the byssus, and 
from that time no motion could be observed in the shell. I 
placed one of these young animals in a valve of Mya trun- 
