68 THE THALLUS Olf THE GENUS PARHELIA. 



former is also provided with absorption pores, the latter 

 has curious slender tufts of contorted paraphyses scattered 

 through the hymenium, that probably serve the same 

 purpose. In S. hispora, where an apothecium rises from 

 the upper cortex, the pseudo-cortex of hyphfe of the lower 

 surface is replaced by a true cellular cortex. 



Heterodea has the thallus of Parmelia and the 

 apothecium of Cladonia. It is of considerable pliylogenetic 

 importance as it forms a link between the two families. 



Taking all the histological factors into consideration, 

 Evernia is best placed with Alectoria., Ramalina, and other 

 fruticose species under Usnacese, as in Zahlbriickneir's 

 classification ; and with these I would also range C'etraria 

 on account of the frequently subpodicellate apothecia, and 

 because of the close relationship of the fruticose species 

 to Usnea. Peltigeracese should be restricted to Peltigera, 

 Nephroma and Solorina, in which the algal symbiont is 

 provided by the Cyanophycese-Poily coccus or Dacty- 

 loooocus ; while Parmeliaoeae should include Sticta (com- 

 prising also Lobaria and Ricasolia), Stictina, Parmelia, 

 Physcia, Pseudo-physcia or Anaptychiai, and Anzia. This 

 last order, ParmeliaceBe, should be divided into two' sec- 

 tions, the first made up of genera with both upper and 

 lower cortex — Sticta, Stictina and Parmelia, the second 

 formed of the remaining genera with upper cortex and 

 lower pseudo-cortex. 



