286 PHYLOGENY 



The thallus varies from crustaceous or minutely squamulose, to lobes of 

 fair dimension in P armeliella and in Hydrothyria venosa, an aquatic lichen. 

 Plectenchyma appears in the upper cortex of both of these, and in the 

 proper margin of the apothecia, while the under surface is frequently provided 

 with rhizoidal filaments. 



These two families form a transition between the gelatinous, and mostly 

 homoiomerous thallus, and the more developed entirely heteromerous thallus 

 of much more advanced structure. The fructification in all of them, gelatinous 

 and non-gelatinous, is a more or less open apothecium, sometimes immar- 

 ginate, and biatorine or lecideine, but often, even in species nearly related 

 to these, it is lecanorine with a thalline amphithecium. Rarely are the spori- 

 ferous bodies sunk in the tissue, with a pseudo-perithecium, as in Phylliscum. 

 It would be difficult to trace advance in all this group on the lines of fruit 

 development. The two genera with bright-green gonidia, Psoroma and 

 Psoromai'ia, have been included in Pannariaceae owing to the very close 

 affinity of Psoroma hypnorum with P armaria rubiginosa ; they are alike in 

 every respect except in their gonidia. Psoromaria is exactly like Psoroma, 

 but with immarginate biatorine apothecia, representing therefore a lower 

 development in that respect. 



These lichens not only mark the transition from gelatinous to non- 

 gelatinous forms, but in some of them there is an interchange of gonidia. 

 The progression in the phylum or phyla has evidently been from blue-green 

 up to some highly evolved forms with bright-green algae, though there may 

 have been, at the beginning, a substitution of blue-green in place of earlier 

 bright-green algae, Phycolichens usurping as itwerethe Archilichen condition. 



e. Peltigeraceae and Stictaceae. The two families just examined 

 marked a great advance which culminated in the lobate aquatic lichen 

 Hydrothyria. This lichen, as Sturgis pointed out, shows affinity with other 

 Pannariaceae in the structure of the single large-celled cortical layer as well 

 as with species of Nephroma (Peltigeraceae). A still closer affinity may be 

 traced with Peltigera in the presence in both plants of veins on the under 

 surface. The capacity oi Peltigera species to grow in damp situations may 

 also be inherited from a form like the submerged Hydrothyria. In both 

 families there are transitions from blue-green to bright-green gonidia, or 

 vice versa, in related species. Thus in Peltigeraceae we find Peltigera con- 

 taining Nostoc in the gonidial zone, with Peltidea which may be regarded 

 as a separate genus, or more naturally as a section of Peltigera; it contains 

 bright-green gonidia, but has cephalodia containing Nostoc associated with 

 its thallus. 



The genus Nephroma is similarly divided into species with a bright-green 

 gonidial zone, chiefly Arctic or Antarctic in distribution, and species with 

 Nostoc (sViO^exms, Nephromium) more numerous and more widely distributed. 



