appearance, which, to an incurious eye, seems like
a petrified fish of about four inches long, the cardo passing for a head
and mouth. It is in reality a bivalve of the Linnaean Genus of Mytilus,
and the species of Crista Galli; called by Lister, _Rastellum_; by
Rumphius, _Ostreum plicatum minus_; by D'Argenville, _Auris Porci_, s.
_Crista Galli_; and by those who make collections, Cock's Comb. Though I
applied to several such in London, I never could meet with an entire
specimen; nor could I ever find in books any engraving from a perfect
one. In the superb museum at Leicester House, permission was given me to
examine for this article; and, though I was disappointed as to the
fossil, I was highly gratified with the sight of several of the shells
themselves in high preservation. This bivalve is only known to inhabit
the Indian Ocean, where it fixes itself to a zoophyte, known by the name
Gorgonia. The curious foldings of the suture the one into the other, the
alternate flutings or grooves, and the curved form of my specimen, are
much easier expressed by the pencil than by words.

_Cornua Ammonis_ are very common about this village. As we were cutting
an inclining path up the Hanger, the labourers found them frequently on
that steep, just under the soil, in the chalk, and of a considerable
size. In the lane above Wall-head, in the way to Emshot, they abound in
the bank in a darkish sort of marl, and are usually very small and soft;
but in Clay's Pond, a little farther on, at the end of the pit, where the
soil is dug out for manure, I have occasionally observed them of large
dimensions, perhaps fourteen or sixteen inches in diameter. But as these
did not consist of firm stone, but were formed of a kind of terra
lapidosa, or hardened clay, as soon as they were exposed to the rains and
frost they mouldered away. These seemed as if they were a very recent
production. In the chalk-pit, at the north-west end of the Hanger, large
nautili are sometimes observed.

In the very thickest strata of our freestone, and at considerable depths,
well-diggers often find large scallops or pectines, having both shells
deeply striated, and ridged and furrowed alternately. They are highly
impregnated with, if not wholly composed of, the stone of the quarry.

LETTER IV.

As in a former letter the freestone of this place has been only mentioned
incidentally, I shall here become more particular.

This stone is in great reque