INTRODUCTION TO CRYPTOGAMTC BOTANY. 883 



in consequence of its disagreeable bitterness. The o-reater 

 portion, however, of the Tripe de Roche, of which so much is 

 said in Sir John Franklin's Narrative of his Overland Journey, 

 was derived from the genus GyropJwra, in which the bitter 

 principle is much more abundantly present, producing diarrhoea 

 and other bad symptoms. Lecanora esculenta and affi^nis 

 are largely used by the inhabitants of the countries east of the 

 Levant, and in some parts of the north of Africa, to mix with 

 wheat in grinding. The portion of nutritive matter which 

 they contain is so much gain to the general stock ; but such 

 an immense quantity of oxalate of lime, nearly 66 per cent, 

 is mixed with it, that the flour which results from the mixture 

 is comparatively innutritions, and to some constitutions, where 

 there is a tendency to what is called the oxalic diathesis, must be 

 very injurious. These species are found scattered over the 

 surface of the ground without any attachment, in the form of 

 rugged truffle-like bodies, of the size of walnuts. Sometimes 

 the bodies are piled together in strata of some inches in thick- 

 ness, in consequence of their being easily rolled along by the 

 wind. The most curious point in their history, however, is, 

 that they are sometimes carried up by whirlwinds, and de- 

 posited after traversing the air for many miles ; giving rise to 

 histories of the miraculous descent of food. A few years since, 

 in a time of great scarcity at Erzeroum, a shower of these 

 Lichens fell most opportunely, to the great relief of the inha- 

 bitants ; and other equally authenticated instances of a similar 

 incident are on record. A pamphlet has been published by 

 Dr. Arthaud to prove that this must have been the manna of 

 the wilderness, with which the Israelites were fed ; but, not 

 to mention its miraculous origin, it does not at all answer to 

 the description in the book of Exodus. Pallas informs us 

 that another species, with very different habits, is eaten by 

 the Kirghi2; Tartars. It forms a crust on the soil and is mixed 

 with Nostoc. Other species besides Cetraria islandica have 

 been used in medicine, and some have found considerable 

 reputation, as Sticta ];>ulmonacea and Peltidea canina. The 

 former owes much of its reputation to the resemblance of the 

 under surface to that of a lung; while the virtues of the latter in 



