6 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
qn 
in a few species it is sometimes areolate. The algal symbionts may be either Cys- 
tococcus or Chroolepus, and the relation between the fungal and the algal symbionts 
is unusually close, the algal cells or clusters being commonly entirely surrounded by 
fungal hyphee or by haustoria, The thallus may be entirely dead and unnoticeable 
or quite prominent even in mature states. 
The apothecia are sessile or more or less sunken in the substratum, or may even be 
covered by a thin layer of the substratum or the thallus. They are usually minute or 
small and most frequently irregular in form, though they may present a rounded con- 
dition and become quite biatoroid in external appearance. Stellate and linear forms 
are quite common, They are black or brownish black in ours, though other colors 
are frequent in other species. The hypothecium is usually brown, but may be 
darker or quite pale. The hymenium is pale, varying to brown, The paraphyses are 
usually branched and frequently scarcely differ from the fungal hyphe in general 
appearance, The asci are commonly of the peculiar pyriform or subpyriform shape. 
The spores have 2, 4, or rarely more numerous cells, and are of the peculiar oblong- 
ovoid form frequently called slipper-shaped or solezeform. They are hyaline, pale, 
or rarely brown or brownish. 
The genus is closely related to Arthothelium, as appears in the structure of the 
apothecia and the nature of the thallus. Perhaps it would be more consistent to sepa- 
rate the 2-celled species under the genus Coniangium Fr.¢ Arthonia properly stands 
next to Arthothelium, but is scarcely higher than Opegrapha and Graphis. 
The genus is southern in range, and only 6 or 7 forms have been found in Minnesota. 
The plants are found on trees, usually species with smooth bark. 
Type species Arthonia versicolor Ach. loc. cit. The plant is uncertain, but the 
plate shows a plant externally like Arthonia radiata (Pers.) Th. Fr. 
KEY TO THE SPECIES, 
Thallus epiphloeodal..............020.000 000202 eee cee eee 4. A. lecideella. 
Thallus mainly or wholly hypophleeodal. 
Spores 2-celled. 
Apothecia rounded...........2...............-... 1. A. patellulata. 
Apothecia varying from rounded to some other form. 
Apothecia rounded to round-oblong........... ‘2. A. converella. 
Apothecia rounded to difform................ 3. A. dispersa. 
Spores 4-celled (rarely more). 
Apothecia tending toward rounded forms........... 6a. A. radiataswartziana. 
Apothecia not rounded. 
Apothecia oblong, difform, or sublinear........ 5, A. punctiformis. 
Apothecia difform, stellate, or ramose......... 6. A. radiata. 
1. Arthonia patellulata Nyl. Nya Bot. Notis. 1853:95. 1853. 
Thallus occurring in patches from 6 to 40 mm. across, White or varying toward sea- 
green or olivaceous, mostly hypophlceodal or quite conspicuous and thick above the 
substratum, sometimes dying away, the thicker conditions usually more or less rough- 
ened; apothecia black, rounded, adnate or immersed, plane, minute or small, 0.3 to 
0.6 mm. in diameter; hypothecium brownish to brownish black; hymenium pale or 
more commonly brownish; paraphyses somewhat gelatinized, but more or less branched 
and scarcely differing from the fungal hyphe; asci broadly clavate to subpyriform; 
spores 2-celled, solezeform, 10 to 14 # long and 3 to 5 pe wide, 
The forms with thick dark thalli may belong to Arthonia ruderalis Nyl. 
4 Vries, Syst. Orb. Veg. 27, 1825. 
bMem. Soc. Sci. Nat. Cherb. 4: 100. 1856. 
