120. 



absence of stipes and podetia ; they are distinguished from the fam- 

 ilies following by the absence of the thalloid exciple. 



There is little doubt that the generic groups of this family indi- 

 cate a polyphyletic origin. We can, however, onl}- state in general, 

 that the several lichen-genera here included are doubtless derived 

 from different fungal ancestors belonging to the Patellariaceae, a 

 discomycetous group of the Ascomycetes (see plate 5). This is 

 evident from the apothecial characters. There is also considerable 

 confusion and uncertainty as to the algal symbionts of the lower 

 groups ; in the majority of species Cystococcus humicola Nag. forms 

 the symbiont, but in some cases Chroolepus umbrina, or at least a re- 

 lated alga, takes its place. Some of the confusion is no doubt 

 caused by the fact that the lower lichens are frequently capable of 

 changing algal symbionts, as already explained elsewhere. Bilim- 

 bia seems to be quite constantly associated with Chroolepus, though 

 I have examined undoubted representatives of this genus which were 

 associated with Cystococcus humicola. 



The following are the general characters of the family : The 

 apothecia are sessile, from small to medium in size ; disk convex or 

 flattened, usually brown or dark in color ; the spores vary greatly 

 in size, form and in the number of septa : the thallus obtains its 

 maximum development in Gyropkora and Umbilicaria, often reach- 

 ing a diameter of several feet. In the lower forms the thallus is 

 sometimes wanting or at least very rudimentary. 



It is essential to call attention to the coloration of the various parts 

 of the lichens belonging to this family. Generic distinctions are 

 based upon the presence or absence of color in the hypothecium, as 

 for example, Lecidea and Biatora, Celidiopsis and Bilimbia. Upon 

 careful investigation it was found that such a distinction is unreliable, 

 as all gradations from a black hypothecium {Lecidea) to a colorless 

 hypothecium {Biatora) can be found. The color of the hypothecium 

 as well as of the thallus, varies considerably with the nature of the 

 substratum ; for the time being, however, the distinctions based 

 upon the color-differences of the hypothecium have been retained 

 (see key). Some experiments on the color-variation in lichens are 

 highly desirable. 



Comparatively few of the Lecideaceae occur upon the soil ; most 

 of them grow upon bark ; some occur upon mosses {Bilimbia) ; and a 

 considerable number upon rocks {Gyropkora and Umbilicaria'). 



