206 



both lichens now continued to exist side by side, undergoing modifi- 

 cations due to the change of environment. The reason for assuming 

 that in Psoroma hyfinorum we have a more suitable symbiotic re- 

 lationship is the fact that this lichen has reached a somewhat higher 

 perfection than Pannaria brunnea (compare plates 69 and 71). It may, 

 however, be possible that the hyphae of one and the same lichen 

 may enter into a symbiotic association with either Cystococcus or 

 Polycoccus. Careful experimentation would decide whether this is 

 possible ; some such experiments are highly important, as they 

 would throw light on the permanency or variability of the algal sym- 

 biont. 



Only two species of Psoroma have so far come to my notice from 

 the territory ; both have been taken from Pannaria ; they are P. hyjb- 

 norum and P. stellata; they occur upon moss or upon the soil, and 

 seem to be somewhat northern in their distribution. 



7. Heppia Naeg. ; Mass. Geneac. Lich. 8. 1854. 



This is a well characterized genus represented by only a few 

 species ; or it may be that there is in reality only one species. H. 

 Desfreuxii, which is the only species so far reported from America,. 

 is, in all respects, identical with the European H. urceolata and H. 

 adglutinata. The following generical characters are, therefore, 

 based upon the study of H. Des-fireuxii. 



The thallus is of medium size, typically foliose, but closely ad- 

 nate to the substratum by means of numerous rhizoids ; it is dis- 

 tinctly lobate at the margin ; the structure of the thallus is peculiar, 

 being almost entirely cortical throughout, the cells extending verti- 

 cally ; there is a thin layer above and below in which the cells are 

 not elongated vertically, and these layers seem to be identical with the 

 cortical layers of other foliose lichens ; there is no medullary tis- 

 sue proper. Almost the entire space between the thin cortical layers 



