, 1891-05-09
Continued from Avis No. So

edge of the mantle of the animal. The
shell itself is a secretion from the invest
ing memb ane of the Mollusc, as will be
subsequently explained; but the pigmen-
tary glands are said to be situated only in
the fringe of the mantle.
Shells are often, perhaps always, clothed in
distinct epidermis or periostracum. This is
of q different colour to the substance of the
shell and is frequently wanting in specimens
from cabinets. Here is a Triton femorale
from Salt Pond covered with its epidermis
and here is another specimen of the same
species from which it has been rubbed off.
This fine she'll from Tortola (Pectunculus
s%ericeus) has a downy epidermis. In some
species the periostracum disappears entirely,
because the animal wears its mantle so as
completely to enfold its shell. Looking about
among the pools at the Fort fat-rocks, you
will sometimes come across what seems like a
beautiful lump of jelly crawling on the sand.
Taking it up you will find that it contains this
shell (Ovulum gibbos um) buried in the folds
of its mantle. The epidermis is of great use to
fresh-water shells, protecting them from the
corroding effect of the carbonic acid in the
water.
Some shells possess a CLASSY epidermis, and
if this be removed bya mineral acid, the sub
stance of the shell, and not merely its internal
coating, is seen to be nacreous. Our common
"Wilk" probably a corruption Of the word
whelk), the Turbo plea, is an example A
pretty ornament, though a spoiled conchologi-
cal specimen, can be made by eating off Its cx-
ternal layer with an acid.
In certain species, the different conchoidal
strata vary and sometimes contrast strikingly
in colour. This is utilised for the manuf.c c.
turo of the gem-like pictures of canoes,
the figure being made to stand out in the
colour of the superficial layer on a deeper
tinted background Of the underlying stras
tum. The West Indian Cassis tuberosa and
Strombus gigas, King and Queen Conchs,
the common ornaments of our houses, are the
favourites of the c~ameo-makers. In thefrst,
white figures are carved out on a claret
ground and in the second, yellow on a
pink.
The colouring of shells is sometimes re-
markably different in the young and adult
stages of growth. Of this the Cypr@a ex-
anthema, common on our beaches, is a fami-
liar and pronounced example. 1t is sOberly,
though handsomely decorated with blush
white bands on a brown ground in the ju-
yenile period of its existence, but covered
when an adult, with a copious but not dis
figuring eruption of discreet small ..pox pus-
tules.
It is true that shells soon lose the fresh
gloss that they had when first taken out of
the water, and that some which have been
kept a long time in the cabinet, have rather
an anomic complexion, as the Doctors might
say. But their specific markings and lines
remain, as a rule, distinct enough through
the ages. Here are q few recent specimens
of the Tellina radiata. Creole children call
it the Aurora shell, and aptly enough, for the
rich lights of a tropical dawn seem to be
photographed on its valves. Sir Robert Schom-
burgh tells of a fossil specimen of the Tellina
radiata from a tertiary deposit in barbados,
which, when first dug out of the rock was
of a homogeneous chocolate colour; but in
which, after it had been exposed for some
hours to the sun, the characteristic fan-shaped
rays of rose and light amber again made their
appearance. The Sleeping Beauty was wax
ened by the kisses of the Royal Sun from the
oblivion, not Of a century, but of countless
@ons, for the tertiary epoch carries us back
to the time when all the World glowed in
the beams of a tropical sun, when tho srhino-
ceros tramped through the bamboo jungles,
crocodiles crawled in the slime, and huge
boa-like pythons coiled round the palm trees
on the land where now stands the Worlds
mighty metropolis-London.
But this is nothing to the case of a Tere-
bratula, a Devonian fossil, now in the Brit
ish Museum, which is marked with hume
rous crimson spots in regular" lines. 1t is
sa;6 hu the wav that red is the most lasting