LINDSAY, ON LECIDEA LUGUBRIS. 
181 
Tlie thallus generally consists of small^ simple^ thick, 
fleshy, convex^ squamules^ with a snb-lobnlate, ill-defined edge. 
The colour is some shade of brown ; in some cases a green 
tint predominating, in others a reddish. Sometimes the 
sqnamnles are so minute, thick, convex, and irregular, that 
they have more the characters of verrucles or thalline warts. 
The under surface of the thallus is usually intimately adhe- 
rent to its basis of support, — which is generally some of the 
primitive or metamorphic rocks, — gneiss in the specimens 
examined by me. It is found, on examination, that the 
surface of the rock has been disintegrated by the lichen, and 
that a sort of union has been formed between them; a union 
so intimate that it is almost impossible to cleanse the under 
surface of the thallus from the adherent particles of fine 
sand — the result of disintegration — without destroying por- 
tions of the cortical and medullary tissues of the thallus. It 
thus becomes difficult to see the black hypothailus upon, 
and from which the brownish thallus is subsequently deve- 
loped. The thallus consists of the ordinary triple division 
of tissues — cortical, gonidic, and medullary. The cortical 
tissue consists of small cellules of a brownish colour. Those 
arranged most externally are somewhat oblong from mutual 
pressure, closely aggregated, and of a deep brown colour ; 
while those placed internally are more round, of a light 
colour, and more loosely aggregated. The gonidia are 
large and distinct, and possess the usual characters. The 
medullary tissue consists of very narrow, delicate, indistinct, 
branching tubes or filaments. 
The apothecia vary in diameter from the -^q to -^^ of an inch. 
They are those of a true Lecidea — being fiat, margined, 
black, and naked ; the thalamium enclosed by a peculiar or 
proper exciple, formed of a substance diff'ering in colour 
from the thallus. The thalamium is open ab initio, and it 
remains flat and open throughout. I have not observed it 
intumescent, so as to become convex or sub-globose, and 
obscuring or covering the margin or exciple; neither does 
the margin swell or disappear, though it may present slight 
irregularities in form. The apothecia are generally round ; 
but they sometimes become, from mutual pressure, when 
two or three are aggregated together, somewhat angular. 
Though sessile, they are not attached, in their mature state, 
by their whole lower surface ; but only by the central por- 
tion thereof. Their base of adhesion is sometimes compara- 
tively narrow, and may become substipitate, or at least may 
raise the lower surface of the apothecium slightly above the 
upper surface of the thallus. The apothecium is sometimes 
Q § 
