INTRODUCTION TO CRYPTOGAMIC BOTANY. 409 



tinctive character consists in their orbicular disc, contained in 

 a distinct excipukim, which is open from the earliest stage of 

 growth, and frequently becomes obliterated by age, in conse- 

 quence of the centrifugal development of that part of the 

 medullary stratum from whence the hymenium is formed. 

 The disc in consequence becomes convex and capitate, assuming 

 the same form as that of Helotium amongst the Pezizm. This 

 transformation sometimes takes place in the most typical 

 genera, as in Lecidea, and is quite normal amongst the nobler 

 species of the group. There is a distinct and regular series, 

 from Lecidea upwards. The crust in that genus is always 

 crustaceous, and often forms merely a thin, close, adherent, 

 fibrous stratum on the hardest flint or quartz. As said above, 

 however, though the margin is well developed at first, it is at 

 length frequently obliterated, so as to conceal from the naked 

 eye the character of the genus. In some species of the tribe, 

 the sporidia are highly developed, as in the New Zealand 

 Biatora marginijiexa (Fig. 79, 6), where they attain a dia- 

 meter of -^l-Q of an inch. Biatora differs but slightly from 

 Lecidea ; but the convexity of the discs, which is accidental in 

 the latter, is normal in the former. In Lecidea the disc is 

 of the same-colour, with the border ; in Biatora, different. The 

 thallus is more inclined to put off the crustaceous form, and a 

 ready link of transition is prepared by it for Ba^omyces. In 

 that genus we have the first indication of a stem to the apo- 

 thecia, or, as it is called, a podetium. Here it is terminated 

 by a single convex and often distorted disc of bright colom-s, as 

 rose, chesnut, &c. The rose-coloured fungoid hymenium of 

 B. roseus and ericetorum are striking ornaments of many a 

 little heathy bank, overshadowed by projecting heath, or other 

 fine-leaved Phrenogams. The thallus is more decidedly 

 foliaceous in Cladonia (Fig. 18, 6), producing free, scale-like 

 fronds, from the midst of which spring cylindrical, or cup- 

 shaped podetia, which are themselves sprinkled with leaves. 

 The margins of the cups, or the tips of the branches bear an 

 abundant crop of convex, irregular, brown, or deep red discs, 

 which are often as bright as sealing-wax. In some cases, where 

 the podetia are strongly branched, the tips alone are fertile and 



