and security of existence by lining its tunnel with a smooth material, a
distinction which cannot fail to impress the observer. In each case the
mollusc is a loose fit in its burrow, having ample room for rotation, but
the aperture of the latter is what is known as a cassinian oval, and
generally projects slightly above the surface of the coral.

The animal is a voluntary life prisoner, for the aperture has the least
dimension of the tunnel. The genus is known to be self luminous--a decided
advantage in so dark and narrow an habitation. It seems to me to be
worthy of special note that an animal enclosed by Nature in tightly
fitting valves should also be endowed with the power of mixing plaster or
secreting the enamel with which its tunnel is lined and of depositing it
with like regularity and, smoothness to that exhibited in its more
personal covering which grows with its growth. The mollusc in its
burrow in the depths of a block of coral, white as marble, with its own
light and its self-constructed independent wall, appeals to my mind as
evidence of the care of Nature for the preservation of types, while from
such retiring yet virile creatures man learns earth-shifting lessons. A
quotation from Lyell's "Principles of Geology" says that the
perforations of Lithophagi in limestone cliffs and in the three upright
columns of the Temple of Jupiter Serapis at Puzzuoli afford conclusive
evidence of changes in the level of sea-coasts in modern times--the
borings of the mollusc prove that the pillars of the temple must have been
depressed to a corresponding depth in the sea, and to have been raised
up again without losing their perpendicularity.

The date-mussels play an important part in the conversion of
sea-contained minerals into dry land. Massive blocks of lime secreted by
coral polyps being weakened by the tunnels of the mussels are the more
easily broken by wave force; and being reduced finally to mud, the lime,