192 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
Thallus usually sea-green. 
Thallus showing open spaces within (inflated). 
Thallus punctured with small holes 
Thallus not punctured with holes 
Thallus not inflated. 
Thallus frequently more or less isidioid above. 
Thallus densely beset above with granules or isidi- 
oid branchlets. 
Thallus margin ciliate; dark and rhizoid-bear- 
ing below. .........00.000.2.2000.2....... 
Thallus margin not ciliate; light-colored and 
rhizoid-bearing below. 
Thallus not densely beset above with granules or 
isidioid branchlets. 
Thallus margin ciliate; dark and densely rhi- 
zoid-bearing below... .-.................. 
Thallus margin not ciliate. 
Thallus sorediate above, dark and rhizoid- 
bearing below. ..............2........ 
Thallus not sorediate above, dark and rhi- 
zoid-bearing below. 
Thallus lobes Jong, not densely 
crowded and imbricated .......... 
Thallus lobes short, densely crowded 
and imbricated............2...... 
Thallus without isidioid branchlets above. 
Thallus rather small, scarcely surpassing 90 mm. in 
diameter; below black and densely rhizoid- 
bearing... 2.2. 2.2...0000 00020 eee. 
Thallus larger, often reaching 15 em. in diameter. 
Thallus margin often ciliate. 
Thallus dark below, where rhizoid-bear- 
ing; apothecia perforate. 
Thallus quite rigid, rugose toward the 
center... 22.2... eee ee 
Thallus less often rugose, thinner 
margin; more often sorediate......- 
Thallus light-colored below; apothecia per- 
forate 
Thallus margin not ciliate. 
Thallus light-colored below, where rhi- 
zoid-bearing .........2.22..2222.202--- 
Thallus dark below. 
Thallus sorediate above, rather small; 
sparingly rhizoid-bearing below.... 
Thallus not sorediate above, except 
sometimes at the margin, much 
larger; strongly — rhizoid - bearing 
below 
Oa. 
10. 
- 
7b. 
Jd 
9. P. pertusa, 
#2. 
=) 
physodes. 
. P. crinita, 
PP. borrert ru- 
decta. 
. P.perlata ciliata, 
P. saxatilis sul- 
cata, 
~ 
. saxatilis. 
~ 
P. saxatilis pan- 
niformis. 
~ 
. tiliacea, 
2. P. perforata. 
. P. cetrata, 
. P. perforata hy- 
potropa. 
3. PP. borrert. 
P. aleurites. 
. £. perlata, 
