_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 of _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_ containing _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_ (subgenus _Nephromium_) more numerous and more widely distributed. _Peltigera_ and _Nephroma_ are also closely related in the character of the fructification. It is a flat non-marginate disc borne on the edge of the thallus: in _Peltigera_ on the upper surface, in _Nephroma_ on the under surface. The remaining genus _Solorina_ contains normally a layer of bright-green algae, but, along with these, there are always present more or fewer _Nostoc_ cells, either in a thin layer as in _S. crocea_ or as cephalodia in others, while, in three species the algae are altogether blue-green. The members of the Peltigeraceae have a thick upper cortex of plectenchyma and in some cases strengthening veins, and long rhizinae on the lower side. Some of the species attain a large size, and, in some, soredia are formed, an evidence of advance, this being a peculiarly lichenoid form of reproduction. The Stictaceae form a parallel but more highly organized family, which also includes closely related bright-green and blue-green series. They are all dorsiventral, but they are mostly attached by a single hold-fast and the lobes in some species suggest the fruticose type in their long narrow form. A wide cortex of plectenchyma protects both the upper and the lower surface and a felt of hairs replaces the rhizinae of other foliose lichens. In the genus _Sticta_ (including the section _Stictina_) special aeration organs, cyphellae or pseudocyphellae, are provided; in _Lobaria_ these are replaced by naked areas which serve the same purpose.