grown tubercles ; but not less frequently the apex is marked
in a much earlier stage with a minute dot, sometimes
whitish as if the nucleus were partially visible, and occasionally
the surrounding part is either flattened or slightly
dimpled, but this is rarely the case ; in general the orifice
expands at length more widely, and often irregularly,
giving the tubercle a broken appearance. Nucleus white,
shrinking in various degrees in drying. When a tubercle
falls off1, the base of the shell, which does not pass under
the nucleus, (and perhaps the lower part of the nucleus
itself,) often remains, forming a white disk with a black
margin; and sometimes the crust rises about the fragment
of a tubercle, and gives it a resemblance to the fructification
of a Thelotrema. Where the whole tubercle has fallen
out, a shallow white impression is left in the crust.
The great similarity of this species to V. gemmata,
t. §617*, has already been noticed in the account of that
Lichen. Its large tubercles and continuous crust distinguish
V. epipolcea from V. muralis ; and it will scarcely be
confounded with any other known British Lichen, except
perhaps the largest state of V. rupestris, a species which
has the base of the tubercle immersed not merely in the
crust but in the stone beneath, and its shell continued, in
some stages at least, under the nucleus. V. pyrenophora
of Acharius, judging from a single specimen, differs but
slightly from V. epipolcea. Its tubercles are truncate, or
concave at the apex, with the margin of the hollow acute
and regularly circular; but a distant approach to such a
conformation is occasionally observable in V. epipolcea.—
W .B .
* Fingerhuth, in his recent Tentamen Floruits Lichenum Eiffliacts, gives
V. alba of Persoon as a “ sufficiently distinct species” from V. gemmata.