211 



by means of long rhizoids ; the margin is lobed but not so dis- 

 tinctly as in Peltigera. The color of the upper surface is brown ; 

 the lower surface is a light brown (S. saccata) , or nearly brick-red 

 (S. crocea). 



The upper cortical layer is smooth above, that is, no trichomatic 

 hyphae are present. This layer is not of uniform thickness ; at 

 certain elongated areas the algae extend almost to the surface, and at 

 these areas the cortical tissue is deficient and the intercellular 

 breathing pores are very numerous. In a vertical section the lower 

 outline of the cortical layer presents a serrate appearance. The al- 

 gal layer is well developed and is peculiar in that two species of al- 

 gae are quite constantly present. The predominating species which 

 occurs nearest the cortical layer is Dactylococcus infusionum ; these 

 algae are most numerous at the above mentioned thin areas. The 

 second species, which is far less abundant, and which occurs just be- 

 low the former, is, perhaps, a species of Polycoccus related to the one 

 found in Peltigera ; it occurs in groups distributed through the 

 medulla and never intermingles with the other form. It is also 

 noticable that in those areas where Polycoccus is quite plentiful, 

 D. infusionum is deficient. 



It seems probable that this group of lichens is now in a transition 

 stage ; that is, it may be assumed theoretically that the original alga 

 was Polycoccus and that the phylogenetic modifications in the fun- 

 gal symbiont are gradually adapting it to enter into a more suit- 

 able symbiotic relationship with D. infusionum ; or it may be pos- 

 sible that the lichen originally entered into a symbiotic association 

 with two species of algae. 



The medullary layer very closely resembles that of Peltigera; 

 the lower hyphal layer is more highly developed ; the hyphal bundles 

 are more numerous and well developed. The reddish color of the 

 lower surface of the thallus is due to a deposit of acid crystals upon 

 the cell-walls of the lower non-cortical limiting layer and the hyphal 

 bundles. 



The general characters of the apothecia resemble those of Pelti- 

 gera ; as a rule they are, however, not marginal but occur toward 

 the middle of the thallus or its lobe ; In S. saccata they occur in 

 depressions of the thallus, much as in Hefflia. The epithecium is 

 dark brown, but in all other respects the apothecial characters closely 

 resemble those of Peltigera. The spore- characters are, however, 



