CHAP. III. 
LICHENS. 
271 
secondly, soredia^ which are heaps of pulverulent bodies scat-^ 
tered over the surface of the thallus. The nomenclature of 
the parts of Lichens has been excessively extended beyond all 
necessity : it is, however, desirable that it should be under- 
stood by those who wish to read the systematic writers upon 
the subject. 
1. Apothecia^ are shields of any kind. 
2. Perithecium, is the part in which the asci are immersed. 
3. Hypothecium ; the substance that surrounds, or overlies the 
perithecium, as in Cladonia. 
4. Scutellum^ is a shield with an elevated rim, formed by the 
thallus. Orhilla^ is the scutellum of Usnea. 
5. Pelta, is a flat shield without any elevated rim, as in the 
genus Peltidea. 
6. Tuberculum, or Cephalodium^ is a convex shield without an 
elevated rim. 
7. Trica^ or Gyroma, is a shield, the surface of which is 
covered with sinuous concentric furrows. 
8. Lirella^ is a linear shield, such as is found in Opegrapha, 
with a channel along its middle. 
9. Patellula ; an orbicular sessile shield, surrounded by a rim 
which is part of itself, and not a production of the thal- 
lus, as in Lecidea. D. C. 
10. Globulus ; a round deciduous shield, formed of the thallus, 
and leaving a hollow when it falls olF, as in Isidium. D. C, 
11. Pilidium ; an orbicular hemispherical shield, the outside of 
which changes to powder, as in Calycium. Z). C. 
12. Podetia ; the stalk-like elongations of the thallus, which 
support the fructification in Cenomyce. 
13. Scypha (oplarium^ Neck.), is a cup-like dilatation of the 
podetium, bearing shields on its margin. 
14. Soredia (ylobuli, glomeruli), are heaps of powdery bodies 
lying upon any part of the surface of the thallus. The 
bodies of which the soredia are composed are called 
conidia by Link, and propagula by others. 
15. Cystula, or Cistella ; a round closed apothecium, filled with 
sporules, adhering to filaments which are arranged like 
rays around a common centre, as in Sphaerophoron. 
16. Pulvinuli, are spongy excrescence-like bodies, sometimes 
