394 MANUAL OF THE MOLLUSCA. 
The boring shell-fish have been distinguished from the mere 
burrowers, perhaps without sufficient reason, for they are found 
in substances of every degree of hardness, from soft mud to 
compact limestone, and the method employed is probably the 
same.* , 
The means by which bivalves perforate stone and timber has 
been the subject of much inquiry, both on account of its phy- 
siological interest, and the desire to obtain some remedy for 
the injuries done to ships, and piers, and breakwaters. The 
ship-worm (feredo) and some allied genera, perforate timber 
only ; whilst the pholas bores into a variety of materials, such 
as chalk, shale, clay, soft sandstone and sandy marl, and 
decomposing gneiss ;} it has also been found boring in the peat 
of submarine forests, in wax, and in amber.{ It is obvious 
that these substances can only be perforated alike by mechanical 
means; either by the foot or by the valves, or both together, 
as in the burrowing shellfish. The pholas shell is rough, like a 
file, and sufficiently hard to abrade limestone; and the animal 
is able to turn from side to side, or even quite round in its cell, 
the interior of which is often annulated with furrows made by 
the spines on the front of the valves. The foot of the pholas is 
very large, filling the great anterior opening of the valves; 
that of the ship-worm is smaller, but surrounded with a thick 
collar, formed by the edges of the mantle, and both are armed 
with a strong epithelium. The foot appears to be a more 
efficient instrument than the shell in one respect, inasmuch as 
its surface may be renewed as fast as it is worn awav.§ (Han- 
cock.) 
The mechanical explanation becomes more difficult in the 
case of another set of shells, lithodomus, gastrochena, sawicava, 
and ungulina, which bore only into calcareous rocks, and attack 
the hardest marble, and still harder shells. (Fig. 25, p. 34). 
In these the valves can render no assistance, as they are smooth, 
and coyered with epidermis; neither does the foot help, being 
small and finger-like, and not appled to the end of the burrow. 
Their power of moyement also is extremely limited, their cells 
not being cylindrical, whilst one of them, sawicava, is fixed in 
* See the admirable memoir by Mr. Albany Hancock, in the An. Nat. Hist. for 
October, 1848. 
+ There is a specimen from the coast of France, in the Brit. Museum, 
} Highgate resin, in the cabinet of Mr. Bowerbank. 
§ The final polish to some steel goods is said to be given by the hands of work- 
women. In Carlisle Castle they point to the rude impression of a hand on the 
dungeon wall, as the work of Fergus M‘Ivor, in the two years of his solitary im- 
prisonment, 
