The association of lichen-forming fungi with blue-green algae may have taken place later in time, or may have been less successful than with the bright-green: they are fewer in number, and the blue-green type of thallus is less highly evolved, though examples of very considerable development are to be found in such genera as _Peltigera_, _Sticta_ or _Nephromium_. _a._ GLOEOLICHENS. Among crustaceous forms the thallus is generally elementary, more especially in the Gloeolichens (Pyrenopsidaceae). The algae of that family, _Gloeocapsa_, _Xanthocapsa_ or _Chroococcus_, are furnished with broad gelatinous sheaths which, in the lichenoid state, are penetrated and traversed by the fungal filaments, a branch hypha generally touching with its tip the algal cell-wall. Under the influence of symbiosis, the algal masses become firmer and more compact, without much alteration in form; algae entirely free from hyphae are often intermingled with the others. Even among Gloeolichens there are signs of advancing development both in the internal structure and in outward form. Lobes free from the substratum, though very minute, appear in the genus _Paulia_, the single species of which comes from Polynesia. Much larger lobes are characteristic of _Thyrea_, a Mediterranean and American genus. The fruticose type, with upright fronds of minute size, also appears in our native genus _Synalissa_. It is still more marked in the coralloid thalli of _Peccania_ and _Phleopeccania_. In most of these genera there is also a distinct tendency to differentiation of tissues, with the gonidia congregating towards the better lighted surfaces. The only cortex formation occurs in the crustaceous genus _Forssellia_ in which, according to Zahlbruckner[1000], it is plectenchymatous above, the thallus being attached below by hyphae penetrating the substratum. In another genus, _Anema_[1001], which is minutely lobate-crustaceous, the internal hyphae form a cellular network in which the algae are immeshed. As regards algal symbionts, the members of this family are polyphyletic in origin. _b._ EPHEBACEAE AND COLLEMACEAE. In Ephebaceae the algae are tufted and filamentous, _Scytonema_, _Stigonema_ or _Rivularia_, the trichomes of which are surrounded by a common gelatinous sheath. The hyphae travel in the sheath alongside the cell-rows, and the symbiotic plant retains the tufted form of the alga as in _Lichina_ with _Rivularia_, _Leptogidium_ with _Scytonema_, and _Ephebe_ with _Stigonema_. The last named lichen forms a tangle of intricate branching filaments about an inch or more in length. The fruticose habit in these plants is an algal characteristic; it has not been acquired as a result of symbiosis, and does not signify any advance in evolution.