200 LICHENS. 



the rocks, he exclaimed, "How beautiful the colouring is ! 

 tell me the cause of those rich tints 1 ' 



The stains were ascribable to four different species 

 of Lepraria, a family where the spores are diffused among 

 the powdery substance of the crust. They are common 

 lichens, and being of such a low state of development, 

 they are not accounted interesting members of their 

 class. 



All lichens are divided into two great classes, the 

 Crustaceous and frondose, the thallus in the first division, 

 consisting of a mere crust spread upon the rock or bark, 

 and partaking all its inequalities; the latter having a 

 leaf)' thallus, the substance outspread in lobed or branch- 

 like fronds. The earlier groups of lichens belong to the 

 Crust Division, with the exception of the Endocarpon, 

 where the thallus expands in lobed fronds. 



A very large family now comes in the regular order ; 

 that of Lecidea. Here we have a decided advance in the 

 scale of development. The thallus or frond becomes 

 somewhat leafy round the circumference, and the fruit 

 cases (Apothecia) are salver-shaped, with a raised border 

 of the same colour as the disk. The substance of the 

 frond is still crustaceous, but it radiates in a regular 

 form round the eds;e. The most remarkable member of 

 this family is the Map Lecidea (L. geographica, Plate 

 XIV., Jig. 2), the thallus is bright yellow, but broken into 

 unequal divisions, by black lines, resembling the divisions 

 between counties in a map, while the black Apothecia?, 

 dotted up and down, and varying in size, resemble the 

 towns and villages there marked. Dr. Murray speaks of 

 this lichen as imparting to the surface of rocks a very 



