Marvels of the Universe 699 
to bear on this beak: so we 
find that it is not rounded as_ 
in most clasp shells, but is 
shaped to something like a 
point, while a further provision 
for wear and tear is made by 
the construction of the hinge 
plate which turns back over 
the beak and is united to 
the shell proper by a series of 
partitions or ribs. To com- 
plete the system of structural 
support a detached and slightly 
curved rib, on the principle of 
a “ flying buttress,” has been 
added ; it curves inside from 
the hinge plate and to it 
the muscles of the mollusc are 
attached. 
It will necessarily be under- 
stood that since the foot is the 
chief instrument used in exca- 
vating these tunnels, it becomes 
an enlarged organ in the Rock- 
borers, and occupies a propor- 
tionately large space within the 
shell. This causes the two valves 
to gape at both ends, a charac- 
teristic very noticeable in all 
burrowing shells, both Rock- 
borers and Mud-burrowers ; 
while it has given a name to 
a section of the latter group eee ~ 7 
By permission of | [Siz H. H. Johnston, G.C.M.G. 
the (Cappers. On the other THE LARGEST WOODPECKER IN THE WORLD. 
hand, the Date Shell, shown to This bird is from twenty-one to twenty-two inches in length. The male has a 
the right of the full-page and curled crimson crest, while that of the female is black. This handsome species is 
. - known as the Imperial Woodpecker. 
again on the opposite page, 
which is also a Rock-borer (although of a distinct species), shows no tendency to gape. The reason 
is that the Date Shell burrows much less deeply than the P.ddocks or the Gapers, so that the foot is 
not abnormally developed and affects only slightly the structure of the shell. But to the average 
bivalve such a gape would mean certain destruction. The Rock-borers, however, are differently 
situated. The gape occurs where little or no strain is felt, and securely lodged as they are in their 
rocky fortresses, they can defy invasion and can neglect with impunity this flaw in their coat of mail. 
One word as to the shape of the shells. The tendency is for the deepest burrowers amongst 
the Borers to have the most cylindrical shells, so we find the Common Piddock is much more elon- 
gated than the Piddock-shaped Borer or the Coral-eater (shown on page 698), and the Date Mussel, 
than any of the true Mussels ; while the Wood Piddock, which is a near ally of the infamous Ship- 
worm, and only buries itself in floating timber to a depth of about one and a half inches—that is, 
just sufficiently to cover the shell—is almost globular in shape, each valve being very deeply moulded. 
50 
