a film and of granulations subsequently formed upon i t : it
is probable, however, that this film is no other than Spiloma
microscopicum, over which the Verrucaria has spread in the
progress of its growth.
This obscure production appears^ like V. niveo-atra, to
have hitherto escaped the notice of every writer on the
Lichens. It differs from that species in the colour and texture
of the crust, and the prominence and more dingy hue of
the tubercles, as well as their more generally conspicuous
nucleus. It seems to bear some affinity to V. leucocephala
in the structure of the tubercle, the shell being thinner and
apparently softer than in the generality of the crustaceous
Verrucarice ; and small and imperfect patches of that species
are so intermixed with all our specimens of V.rudis
upon bark, as almost to lead to a suspicion whether the two
be really distinct. Yet the tubercles are not powdered,
nor do they partake at all of the tendency to a cylindrical
figure so observable both in the denudated state and in the
more common appearance of V. leucocephala; and they
differ further by their minute size, rugged surface, and
hardly discoverable orifice: the nature of the crust, too,
seems essentially different. To distinguish V. rudis in its
palest state from V. biformis and V. olivacea, it is only necessary
to advert to the more even crust and the larger and
more distinctly perforated tubercles of both those species..
W. B.