(54) 
cannot separate the latter, surely the obsolescence of the hypothallus will 
not alone serve to distinguish generically the, in every other respect, sim- 
ilar L. asperellum, Th. Fr., which, according to Nylander (Lich. Scand. 
p. 25) is a Pterygiwm. From this species, Pterygium Petersiit and 
P. centrifugum, Nyl., chiefly differ, as respects the thallus, in being more 
evidently foliaceous, and (as a consequence of this?) in exhibiting more 
distinctly the very regular, elongated cells which constitute the marked 
medullary tissue.——P. nigra (Huds.) Nyl. (Collema, Ach., Lecothecium, 
Mass., & Auctt.). Calcareous rocks, and sandstones, throughout the 
United States. Canada (A. T. Drummond). Virginia (Curtis). Ala- 
bama (Peters). Thallus parenchymatous throughout. Apothecia now 
reddish-brown (Trenton Falls, N. Y.) but more commonly black. Spores 
(of the Trenton lichen) ellipsoid, bilocular, or at length more oblong and 
3-4-locular, 0,011-16™- long, and 0,0045-0,006™"- wide. In the same 
Trenton lichen the approach appears to be a gradual one to var. cesia, 
Nyl. (Collolechia, Mass., & Auctt.) wherein, through what seems a chem- 
ical change, the external colour alike and the normal structure of the 
thallus are modified. The extent of this degradation is, however, various 
in the European specimens, as is seen in comparing Nyl. Herb. Par. n. 115 
with Anz. Lich. Ital. Sup. n. 10; and in the latter at least there is no 
difficulty in observing the parenchyma of the thallus. Spores of this 
variety, from Trenton, ellipsoid, or biscoctiform, bilocular, 0,011-14™™ 
long. and 0,007-8™™ wide. The external resemblance of P. nigra to 
P. tryptophylla is no doubt corroborated, to a certain extent, by the ana- 
tomical structure of these lichens; but we cannot well separate the former 
from the group with which it is here associated, and this group diverges 
from that which includes P. tryptophylla, in its spore-character; to say 
nothing of other differences.——P. flabellosa, Tuckerm. (Obs. Lich. 1. ¢. 
5, p. 401) Granitie rocks, Vermont (C. C. Frost). Sufficiently distinguish- 
able externally from the last, especially by its marginal lobes, and indis- 
tinct hypothallus. As regards internal structure the two plants are gen- 
erally alike. In a more luxuriant but infertile, similar lichen collected by 
me, on granite, in the White Mountains, the lobes of the circumference 
exhibit however, under the microscope, a compact medulla comparable 
only with that of P. Petersii ; and the plant before us may be said to be, 
in several respects, intermediate between that species and P. nigra. 
Spores oblong-ellipsoid, 4-locular, 0,016-18™™- long, and 0,0055-7™™- 
wide. Lecothecium adglutinatum, Anz. (Manip. n. 12, Lich. Langob. n. 
268) from granitic rocks in Upper Italy, seems to be well comparable 
with our plant.— P. Petersii,, Tuckerm., is only known to me from its 
' Pannaria Petersii: thallo parvulo membranaceo stellato-expanso e¢ lirido- 
glauco olivaceo-nigrescente, laciniis appressis plano-conrexris centro squamuloso- 
dispersis delabentibus ambitu radiantibus laciniato-multifidis, hypothallo obsoles- 
cente ; apotheciis minutis pseudo-biatorinis nigris, margine tenui demum suber- 
