(91) 
of thallus, as exemplified especially in C. aggregatum, with C. callibotrys, 
Tuck., and C. coccophylloides, Nyl.; however marked may well, at first, 
appear the difference in the spores. The real significance of the group 
represented by C. callibotrys is not however to be got from depauperate 
exhibitions of it (C. quadratum, Lahm) or from anything less than a 
complete view of its spore-history. Nor is it, we will venture to say, 
other than likely that the elongation of the spore in C. aggregatum, &c., 
represents only an extreme, to be explained hereafter, in the course of 
discovery, by the mediation of forms with much shorter spores; exactly 
asin C. flaccidwm. In that case it is possible that the present cluster 
shall prove in fact too near to the one immediately preceding. 10. C. 
microptychium, Tuck. Lich. Calif. p. 35. Trunks of Elm, Chestnut, and 
other trees; New England. Related by the spores to C. leptaleum, but 
the thallus is widely diverse, and looks rather towards the next.—— 
ll. C. flaccidum, Ach. Granitic rocks, and also on trunks, common in the 
north, and in the mountains southward, to Virginia. In tbe Carolinas, 
infertile (Mr. Ravenel). The spores, in European specimens of this 
species (as in Scher. 413, 414, and Moug. & Nestl. 1059, and as figured by 
Hepp, 651) are very commonly ovoid, and offer little to distinguish them 
from conditions of the type in Collema proper; especially when, as I 
observe in Zw. exs. 166, this ovoid spore clearly betrays a 2isws to become 
muriform-plurilocular. And this sufficiently explains Nylander’s relega- 
tion of C. flaccidum (Syn.) to the neighbourhood of C. furvum; and his 
more recent denial (Lich. Scand.) of any appreciable diversity between 
the spores of these two lichens. Butif the lichen-group before us be 
indeed so far determinable as a ‘“‘ Collema,” it is none the less certain that 
its ulterior development is that of ‘' Synechoblastus;” or that the alleged 
spore-difference, upon which it has been sought to construct even a gener- 
ical distinction, disappears thus entirely within the circuit of modifica- 
tions of a single species. The gradual evolution of the ovoid into long 
fusiform spores is sufficiently exhibited even by the European specimens, 
and the contrast between Hepp’s figure already cited and that of Massa- 
longo (Mem. n. 109) seems in fact greater than the measurements 
express: in our lichen however the elongation of the spore is commonly 
much more pronounced than in Massalongo’s; and we cease finally to 
find any criterion of distinction, in this regard, from C. nigrescens. The 
latter is indeed closely approached, in all respects, by some of our tree- 
forms of the present; and the two species belong clearly to one and the 
same cluster. ——Of the plants referred to C. abbreviatum, Arn., 1. ¢., 
the writer possesses only Scher. 413, 414, which have not afforded to him 
any sufficient differences from Zw. 166, or from C. flaccidum. But even 
the perplexing spores figured by Arnold (1. ¢. t. 4, n. 77-80) are no more 
difficult to admit (compare the same writer’s n. 74) as an element of C. 
jlaccidum, than that lichens so generally similar as those just cited, of 
Scherer and v. Zwackh, should not at any rate be members of one and 
