_cc._ _CLADONIACEAE._ This last phylum of Lecideales is the most interesting as it is the most complicated. It possesses a primary, generally sterile, thallus which is dorsiventral and crustaceous, squamulose or in some instances almost foliaceous, along with a secondary thallus of upright radiate structure and of very varied form, known as the podetium which bears at the summit the fertile organs. A double thallus has been suggested in the spreading base, containing gonidia, of some radiate lichens such as _Roccella_, but the upright portion of such lichens, though analogous, is not homologous with that of Cladoniaceae. The algal cells of the family belong to the Protococcaceae. Blue-green algae are associated in the cephalodia of _Pilophorus_ and _Stereocaulon_. The primary thallus is a feature of all the members, though sometimes very slight and very short-lived, as in _Stereocaulon_ or in the section _Cladina_ of the genus _Cladonia_. Where the primary thallus is most largely developed, the secondary (the podetium) is less prominent. This secondary thallus originates in two different ways: (1) the primary granule may grow upward, the whole of the tissues taking part in the new development; or (2) the origin may be endogenous and proceed from the hyphae only of the gonidial zone: these push upwards in a compact fascicle, as in the apothecial development of _Lecidea_, but instead of spreading outward on reaching the surface, they continue to grow in a vertical direction and form the podetium. In origin this is an apothecial stalk, but generally it is clothed with gonidial tissue. The gonidia may travel upwards from the base or they may possibly be wind borne from the open. The podetium thus takes on an assimilative function and is a secondary thallus. The same type of apothecium is common to all the genera; the spores are colourless and mostly simple, but there are also changes in form and septation not commensurate with thalline advance, as has been already noted. Thus in _Gomphillus_, with primitive thallus and podetium, the spores are long and narrow with about 100 divisions. 1. ORIGIN OF CLADONIA. There is no difficulty in deriving Cladoniaceae from _Lecidea_, or, more exactly, from some crustaceous species of the section _Biatora_ in which the apothecia—as in Cladoniaceae—are waxy and more or less light-coloured and without a thalline margin. In only a very few isolated instances has a thalline margin grown round the _Cladonia_ fruit. There are ten genera included in the Cladoniaceae, of which five are British. Considerable study has been devoted to the elucidation of developmental problems within the family by various workers, more especially in the large and varied genus _Cladonia_ which is complicated by the presence of the two thalli. The family is monophyletic in origin, though many subordinate phyla appear later.