slightly zonate edge. The surface is unpolished, sometimes
continuous, sometimes full of narrow cracks : when wet it
is grey, variously tinged with green ; when dry usually almost
white and looking as if powdered ; under some circumstances
nearly bare, and of a dark olive grey. The
internal substance is green. The tubercles, when young,
are often so completely covered with loose white powder,
as to resemble the pulvinuli of a Spiloma. When fully
formed, they are usually contracted at the base, spherical,
ovate, or even elongated like the papillulae of an Isidium.
The shell is brown under the pruinose film, which invests
it, in general, even in the advanced state of the tubercle,
except at the very apex. Occasionally it is bare, and then
of a blacker hue. The Lichen calliculosus of Hoffmann,
Enum.p. 17. t. 2 . f 2, which Acharius quotes as a synonym,
is figured and described with somewhat hairy tubercles.
Our (3 {fig. b.) is, in itself, a slight variety, with larger,
clustered, often irregular tubercles : but we have specimens
on old bark from Mr. Lyell and from Mr. Robertson, on
which, among the tubercles, as they appear to us, of V. leu-
cocephala, are found patellulae of a dull purplish black, covered
with an inseparable superficial buff powder. See our
Jig. c. Such specimens, perhaps, suggested to Acharius the
trivial name amphibolus, and induced him, subsequently to
the publication of his Synopsis, to transfer the species to his
new genus Cyphelium*, where it surely finds, however, very
unmeet associates in the sessile-fruited Calicia, from which
the genus was originally constructed. Precisely similar
patellulae occur on sand-rocks in Sussex, intermixed with
what may possibly be abortive tubercles of the Verrucaria.
Whilst we know no other Lichen to which these patellulae
can be referred, we believe, but cannot positively affirm,
that they do not belong to our Verrucaria. How incessantly
do the lectures of Nature admonish her students to humility
! How imperfectly can we trace even the lowest operation
of the Almighty hand !—W.B.
* I have not access at present to the figures in the Stockholm Transactions.