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sterigmatibus fere semper multi-articulatis. Thallus crustaceus, 
aut effiguratus aut rarissime suffruticulosus aut uniformis, sepius 
flavescens. 
The lemon-coloured Placodia make part, as Fries remarked (L. E. 
p. 114) of but a single series, which, beginning with Theloschistes (the 
lemon-coloured group of Parmelia-Imbricaria, Fr.) ends with the analo- 
gous section of the (granulose) Patellarie. But notwithstanding the many 
difficulties, noted also by Koerber (Syst. p. 110) — who, it is observable, 
distributes, at the place cited, the Gyalolechie also among the two groups 
to which he refers almost the whole of our Placodium — most lichenogra- 
phers have agreed in recognizing a sufficiently marked distinctness of 
texture, which, taken in connexion with the whole history of the devel- 
opment of its nearest allies, refers Lichen elegans to the crustaceous, and 
L. parietinus to the foliaceous families. And the difference is certainly 
less between the two extremes of the crustaceous thallus, than between 
the highest forms of this and the foliaceous. Taken as a whole, Placo- 
dium, as here understood, is well-marked by the predominant coloration 
of both thallus and apothecia; as by the tun-shaped, polar-bilocular 
spores. To the character first named there are yet some exceptions, 
which assume the rank of genera in many works. But Pyrenodesmia, 
Massal., differs, as Koerber remarked, in nothing but colour from his 
Callopisma; and the distinction is still more obviously inadequate to 
separate the American, arboricoline P. camptidium and P. Floridanum 
from the same group with the otherwise closely related P. ferruginewm. 
And we are thus not without plain indications, that however distinguished 
by the predominance of species of the lemon-colored series, the genus is 
by no means confined to it. The thalline exciple is often distinct enough, 
and predicable of all the species; but it disappears, sometimes almost 
from the first in the granulose section, when the often marginate disk 
assumes the whole aspect of Biatora. Yet we find positively no real dif- 
ference of structure between the at length pseudo-biatorine apothecia of 
P. aurantiacum (Callopisma, Auctt.) and the so-called biatorine ones of 
P. ferrugineum and P. sinapispermum (Blastenia, Auctt.).——There is 
in general no safer criterion in the present group than that afforded by the 
spores. We find these varying however even in P. vitellinwm to obsoletely 
bilocular, and even simple; and the difference is only one of degree between 
such spores and those of other species, referable to Gyalolechia of authors. 
And if P. fulgens, DC. (Fulgensia, Massal.) be found, as Anzi (Catal. 
Sondr. p. 46, with which compare also Th. Fr. Lich. Arct. p. 81) describes 
it, with all the characters of Gyalolechia bracteata, excepting only that 
the spores are simple, one might well incline to assume that those of the 
Gyalolechia are as properly describable as sometimes simple, and to 
restore both these most closely allied lichens, as more or less aberrant 
forms, to their ancient and natural associations. 
