urvives, than the shell which it covered; nay, it seems
not impossible that to its thick epidermis, originally an essentially
different substance from that which composed the calcareous case, the
shell may have owed its preservation as a fossil; while other shells,
its contemporaries, from the circumstance of their having been
unfurnished with any such covering, may have failed to leave any trace
of their existence behind them. It seems at least difficult to conceive
of a sea inhabited by many genera of fishes, each divided into several
species, and yet furnished with but one species of shell. I found the
quarry of Pickoquoy,--a deep excavation only a few yards beyond the
high-water mark, and some two or three yards under the high-water
level,--deserted by the quarrymen, and filled to the brim by the
overflowing of a small stream. I succeeded, however, in detecting its
shells _in situ_. They seem restricted chiefly to a single stratum,
scarcely half an inch in thickness, and lie, not thinly scattered over
the platform which they occupy, but impinging on each other, like all
the gregarious shells, in thickly-set groups and clusters. There occur
among them occasional scales of Dipteri; and on some of the fragments of
rock long exposed around the quarry-mouth to the weather I found them
assuming a pale nacreous gloss,--an effect, it is not improbable, of
their still retaining, attached to the epidermis, a thin film of the
original shell. The world's history must be vastly more voluminous now,
and greatly more varied in its contents, than when the stratum which
they occupy formed the upper layer of a muddy sea-bottom, and they
opened their valves by myriads, to prey on the organic atoms which
formed their food, or shut them again, startled by the shadow of the
Dipterus, as he descended from the upper depths of the water to prey
upon them in turn. The palate of this ancient ganoid is furnished with a
curious dental apparatus, formed apparenly, like that of the recent
wolf-fish, for the purpose of crushing shells.

About mid-day I set out by the mail-gig for Stromness. For the first few
miles the road winds through a bare solitary valley, overlooked by
ungainly heath-covered hills of no great altitude, though quite tall
enough to prevent the traveller from seeing anything but themselves. As
he passes on, the valley opens in front on an arm of the sea, over which
the range of hills on the right abruptly terminates, while that o