posit of acid crystals (vulpinic acid) in and upon the cells of the 

 outer semicortical layer. Soredia and isidioid branches are also 

 frequently present, particularly in E. vulpina and E. fnrfuracca. 



The general structure of the thallus is as follows : The outer- 

 most protective covering consists of an irregular network of hyphal 

 cells which usually extend in a direction vertical to the outer surface ; 

 the cell-walls are much gelatinized and lined with the above men- 

 tioned deposit of acid crystals ; next follows the algal layer, which 

 consists of Cystococcus hnniicola surrounded by the haustorial 

 branches ; the algae, while few in number, are comparatively large, 

 no doubt due to hypernutrition ; still more internally is found the 

 medullary layer, usually associated with the special mechanical tis- 

 sue ; the central area is either hollow, or filled with a loose hyphal 

 tissue, the cells of which are more or less interwoven ; the mechan- 

 ical tissue consists of hyphal bundles, the cells of which extend par- 

 allel with the longitudinal axis of the thallus ; these bundles occur 

 at regular intervals, forming a broken ring as seen in cross section ; 

 sometimes this ring becomes more or less continuous, approaching 

 in structure the hollow cylinder ; the mechanical tissue is especially 

 adapted to resist both longitudinal (pulling) and lateral tensions. 

 In some species this special mechanical tissue is wanting, the nec- 

 essary mechanical tissue being supplied by the external cortical layer, 

 while the entire interior is filled by the loose medullary tissue. 



The color of the thallus varies from greenish grey (E. furftir- 

 acea) to brilliant yellow {E. vnlpina). Various fungal parasites 

 are not uncommon upon the thallus of E. vnlpina and other species 

 besides the spermagonia. 



The apothecia are usually terminal, or nearly so, large, the disk 

 flattened with a more or less irregular outline; the margin (excipu- 

 lar) often bears small thalloid branches ; the thecium is brown to 

 dark brown. The paraphyses and spore-sacs are much as in Cetra- 

 ria, likewise the spores. The hypothecium is colorless and usually 

 consists of two distinct layers. 



The Evernias are typically western, most of them being limited 

 to the Rocky mountains and Pacific coast region, where they occur 

 upon trees and rocks. Only the sterile form of E. viilpina occurs 

 in our territory. 



