166 



Because of the variability of P. saxatilis, and because it is 

 often sterile, it presents difficulties in conjunction with other 

 species of this group, which are best solved by learning the typical 

 forms of the long-recognized species, rather than by giving names 

 to the confusing intermediate forms. Yet some of its variations 

 are so striking in appearance that they require separate descrip- 

 tion to make identification possible. P. saxatilis is further dis- 

 cussed in relation to P. tiliacea under that species. 



Pannelia sulcata. Furrowed Shield Lichen 



Also called P. saxatilis var. sulcata, or var. rosaeformis. Though 

 actually only a phase of P. saxatilis, this lichen has an appearance 

 so striking and so easily recognized, that it may be treated as a 

 subspecies for the sake of emphasis. It is larger, sometimes 20 cm. 

 across, and with branches 1 cm. wide. The margins and ridges 

 are studded with raised, dusty, white soredia, lengthened or ranged 

 in lines accenting the net-like pattern, In the rare cases where 

 fruits are found, soredia may cover the rims. In no other respect 

 does it differ from P. saxatilis. 



Somewhat similar arrangements of soredia are seen on Sticta 

 piilmonaria (Group 7), but that species has the under surface 

 nearly white, and the pits are much deeper, with rounded, instead 

 of angular ridges between. Pannelia sulcata is not easily mistaken 

 for any other lichen. 



Pannelia frondifera 



A name recently given to a rare, freakish development of 

 P. saxatilis, in which the tiny prongs of coral-like growth char- 

 acteristic of var. furfur acea become flattened into miniature lobes 

 more or less covering the lichen. This modification is probably 

 no more worthy of being named a species than are the fasciated 

 forms of some higher plants, but it does give a decidedly dif- 

 ferent appearance suggesting Physcia aqiiila var. dctonsa (Group 

 8), which, however, is moss-green or brownish, and Pannaria 

 microphylla (Group 10) which is greenish l^rown. Both these 

 species have a dull surface, contrasting with the smooth and some- 

 what shining texture of Pannelia frondifera. 



