(107 ) 
Placodium differs from the other genera of this sub-family in its multi- 
articulate sterigmas (arthrosterigmata, Nyl.) the most complex form which 
this structure assumes; pointing also, as do so many other features of the 
genus, towards Theloschistes. 
The range of the group is decidedly northern: but not a few forms 
recur, under the suitable atmospherical conditions, in the warmer regions 
of the earth; and others, described principally by Nylander (in Prodr. 
Fl. N. Gran. p. 28) are confined to tropical countries. Of the forty odd 
best known species, not quite half have been found as yet to occur in 
North America; and the relative proportion of the North American to 
the European is about the same. 
The fruticulose exaltation of the crustaceous thallus, though perhaps 
more remarkable than the sub-foliaceous one, has received much less 
attention. Lecanora fruticulosa, Eversm., from the Kirguis deserts, has 
indeed long been known in the memoir in which it was illustrated, but 
specimens are rare; and no other instance of the kind (if we except Le- 
cidea conglomerata and L. vesicularis) had occurred till the discovery of 
the Californian Lecanora Bolanderi. The little group is now increased 
by the addition, from the same region of North-western America, 
of two fruticulose Placodia. The terete and solid thallus of these is 
as properly crustaceous as that of the fruticulose Lecanore, and so far 
diverse from all fruticulose expressions of Theloschistes; but the two 
lichens differ from each other more than do the analogous Californian 
conditions of Lecanora, though perhaps equally conceivable as illustra- 
tions of a single type (Thamnoma). P.(Thamnoma) coralloides, Tuck- 
erm. Obs. Lich. 1. c. 6, p. 287, though distantly comparable (so far as the 
few specimens go) in its decumbent habit, as well as in colour, with The- 
loschistes chrysophthalmus v. flavicans, is yet a crustaceous lichen; and 
the simply bilocular spores, showing no indications of an isthmus, rather 
resemble, except that they occur only in eights, those of P. vitellinum. 
——And the affinity of the other species to the present genus is still more 
unmistakable. The erect, fastigiately-branched trunks of P. cladodes, 
described at the place just cited, are densely crowded together, and their 
papilleform tips constitute a warted crust, with much the habit of that 
of some granulose Lecanora, or Placodiwm, and the colour of P. elegans. 
P. fulgens, DC., has occurred, in its perfect, subfoliaceous condi- 
tion, on calcareous earth, in the bad lands of Judith, Nebraska; as also 
on the North Platte, nearer to the Rocky Mountains (Dr. Hayden) and in 
Montana (Mr. M. A. Brown) accompanied by the granulose v. bracteatum. 
of authors. The spores (of the variety) though often simple, occur also 
in variously imperfect bilocular conditions; but I observe not wholly dis- 
similar spores in some of my foreign specimens of a; and both forms may 
perhaps well be kept together as varying states of a single, so far aberrant, 
Placodium. A granulose condition of this species (v. alpinum, Th. Fr.) 
exceedingly near, as described, to the other, has occurred in Greenland 
