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tissue, and more distinctly concatenated gonidia, between it and P. rabig- 
inosa, and the latter offering seemingly the first step of descent from the 
compacter, filamentous web of P. molybd@a and P. plumbea. And yet, 
if continuing the investigation, we compare the species first named, thus 
both by internal and external characters associated with Pannaria, with 
Collema byrse@umn (C. byrsinum, Ach.) significantly agreeing with it, toa 
considerable degree, in habit, as in the important point of the nap of 
the under side of the thallus, and in the spores, it may well appear doubt- 
ful whether Montagne had not equal right ; and whether there be really, 
here, any distinction at all between Punnaria and Collema. 
And the difficulty returns, under varied conditions, in the reduced 
forms, which make so large, and, in the north at least, so characteristical 
apart of Pannaria. It were indeed to be expected beforehand, in view 
of the evident approaches,—to say the least—in the higher types, 
towards Collemaceous structure, that such approaches, rendered yet more 
erplexing by the degradation of the thallus, should recur in the lower. 
Lecothecium, Trev., Racoblenna and Collolechia, Mass., Pterygium, Nyl., 
and Wilmsia, Koerb., are modern genera of Collemacee, every one of 
which may, notwithstanding, with fair show of reason be said, not merely 
to descend from, but even to be referable to Paxnariei. Not by any 
means that a certain degree of structural change in the thallus is not 
recognizable in these groups, or this group, but that this change is, to a 
very great extent, —unless where chemical conditions may possibly have 
to come into account, as in Collolechia —inextricably involved in, and it 
should seem, in snort, a corollary of, that reduction of the thallus,! which, 
confessedly, is not enough in itself to exclude any lichen from Pannaria. 
The modified structure follows the reduction, in fact, within the univer- 
sally recognized limits of the genus. What is taken as sufficient to 
separate Lecotheciium nigrum, Mass., (Pannaria, Nv.) and L. asperellum, 
Th. Fr. (Lich. Arct., and herb. Auct.,—the plant being comparable per- 
laps rather with Pterygiwm pannariellim, Nyl. Scand., than with this 
author’s P. asperellum, 1. ¢.) from true Lichens, is not indistinctly trace- 
able to an unquestioned lichen—Pannaria tryptophylla; and even 
Pterygium Petersti, Nvl., analogous as are its structural details to those 
' Compare Schwendener on the structure of the ‘ smaller squamules,’ as con- 
trasted with the larger, of P. microphylla, 1. c. 3, p. 194, and on the thallus of 
Racoblenna Tremniaca, Mass., and Lecothecium corallinoides, Koerb. (Pannaria 
nigra, Nyl.) 1. c. 4, pp. 162, 165. And his observation of the reduction of the 
thallus of 2. microphylla into « ‘through and through’ parenchymatous tissue, 
holds good equally of P. tryptophylla ; which, though, on the one hand, compared 
by the German author with even P. rubiginosa, exhibits also, on the other, condi- 
tions, inseparable in thalline structure from P. nigra. But in reaching this 
merely parenchymatous thallus, Pannaria reaches ultimately Pyrenopsis and the 
Collemeci ; all other distinction between the two families, as now understood, at 
last ceasing, except what may be made of the external habit. 
