PANDORA.
27
difference of locality. When the littoral zone is sandy,
the surface is apt to be disturbed by waves and occasional
storms, so that the stratum may be of a greater or less
thickness at one time than at another : now it is covered
by a deposit of material thrown up by the sea ; in a few
days this cover may be stript off. In order to prevent its
tubes being choked by an accumulation of the imported
material, the Pandora living between tide-marks gradually lengthens that end of its shell. The variety which
inhabits deeper water is not exposed to fluctuations of
this kind ; it therefore does not require any such provision, and lies undisturbed in its level bed. This may
explain the variation in the proportions of length and
])readth which is exhibited by the two forms. The difference of thickness in the shells of P. incequivalvis and
its varieties also depends on habitation. I am inclined
to think that, with regard to every species living ])oth in
the littoral and coralline zones, the shell is thicker in
the former and thinner in the latter.
Examples to
illustrate this proposition occur in Venus gallina and its
varieties striatula and laminosttj Mactra solida and its
variety elliptica, Trochus zizij)hinus and its small conical
variety, Buccinum undatum and its variety Zetlandica,
and in many other species. Experiments made by Dr.
Da\y, Forchhammer, and BischofF have proved that the
quantity of carbonate of lime held in solution by seawater, and from which shells are produced, is greater on
the coast than in the ocean ; it is derived from the land,
and brought down to the sea by rivers and streams, the
washings of rain, and the action of waves. This fact
ought not to be lost sight of in discriminating species
from varieties of which the comparative solidity and
size are the sole or chief criteria.
Lamarck at first named this species P. margaritacea,
c2
27
difference of locality. When the littoral zone is sandy,
the surface is apt to be disturbed by waves and occasional
storms, so that the stratum may be of a greater or less
thickness at one time than at another : now it is covered
by a deposit of material thrown up by the sea ; in a few
days this cover may be stript off. In order to prevent its
tubes being choked by an accumulation of the imported
material, the Pandora living between tide-marks gradually lengthens that end of its shell. The variety which
inhabits deeper water is not exposed to fluctuations of
this kind ; it therefore does not require any such provision, and lies undisturbed in its level bed. This may
explain the variation in the proportions of length and
])readth which is exhibited by the two forms. The difference of thickness in the shells of P. incequivalvis and
its varieties also depends on habitation. I am inclined
to think that, with regard to every species living ])oth in
the littoral and coralline zones, the shell is thicker in
the former and thinner in the latter.
Examples to
illustrate this proposition occur in Venus gallina and its
varieties striatula and laminosttj Mactra solida and its
variety elliptica, Trochus zizij)hinus and its small conical
variety, Buccinum undatum and its variety Zetlandica,
and in many other species. Experiments made by Dr.
Da\y, Forchhammer, and BischofF have proved that the
quantity of carbonate of lime held in solution by seawater, and from which shells are produced, is greater on
the coast than in the ocean ; it is derived from the land,
and brought down to the sea by rivers and streams, the
washings of rain, and the action of waves. This fact
ought not to be lost sight of in discriminating species
from varieties of which the comparative solidity and
size are the sole or chief criteria.
Lamarck at first named this species P. margaritacea,
c2