INTRODUCTION TO CRYPTOGAMIC BOTANY. 403 



discovered by Hooker and Thomson, in Khasya, on the leaves 

 of a species of Symiilocos* In most genera the individual 

 perithecia are much elongated, and often pointed at either end, 

 and when these are parallel, or placed at different angles to 

 each other, they afford groups very similar to those of Chinese 

 or Japanese letters. The characters on which the different 

 genera depend, are founded principally on the nature of the 

 perithecium, which is more or less developed according to 

 circumstances, till in Arthonia, properly so called, for there 

 are many spurious species, it vanishes almost entirely. In 

 a few instances the linear or branched nature of the peri- 

 thecia is lost, and they resemble Hysterium (Fig. 66, a). 

 Sometimes the perithecium is entirely confined to the base, 

 sometimes it forms a lateral wall all round, being deficient 

 above and below, and sometimes it is completely restricted to 

 the upper surface ; and, according to circumstances, the disc, is 

 more or less fully exposed and distinct from the surrounding 

 tissue. 



Tulasne has figured in Opegrapha minute scattered sper- 

 matogonia, sunk in the substance of the crust, with linear 

 moderately long spermatia. A few species of Grapliidei occur 

 in the frigid and temperate zones of both hemispheres ; but by 

 far the greater part are found in tropical America, which has 

 several genera or subgenera peculiar to itself It is remarkable 

 that only one Graphis, and not a single Opegrapha, has yet 

 been found in New Zealand. Sclerophytonf is worth mention 

 because of its elliptico-linear apothecia, with their coloured disc, 

 being disposed in a single row on linear elevations of the 

 crust, so that it answers to Trypethelmm. 



4. Pyxinei, Fr. 



Disc orbicular. Excipulum distinct from the thallus, at 

 first closed, superficial. Thallus horizontal, foliaceous, for the 

 most part fixed by the centre. 



441. This is one of the most singular groups of Lichens, 

 both as regards the superficial fruit and the curious convolu- 

 tions presented by the hymenivmi of many of the species. The 



* Gardeners' Chronicle, 1854, p. 217. 

 t Moutagne, iu Ann. d. Sc. Nat., s6r. 3, vol. 16, tab. 16. 

 26 * 



