=326. Nomenclature.= The need of short distinctive names of international value for plant formations is obvious; it has become imperative that successions also should be distinguished critically and designated clearly. From the very nature of the case, it is impossible to designate each formation or succession by a single Greek or Latin term, as habitats of the same character will show in different parts of the world a vegetation taxonomically very different. It may some day be possible to use a binomial or trinomial for this purpose, somewhat after the fashion of taxonomy, in which the habitat name will represent the generic idea as applied to formations, and a term drawn from the floristic impress the specific idea. Such an attempt would be futile or valueless at the present time; it could not possibly meet with success until there is more uniformity in the concept of the formation, and until there has been much accurate and thorough investigation of actual formations, a task as yet barely begun. At present, it seems most feasible as well as scientific to designate all formations occupying similar habitats by a name drawn from the character of the latter, such as a meadow formation, _poium_, a forest formation, _hylium_, a desert formation, _eremium_, etc. A particular formation is best designated by using the generic name of one or two of its most important species in conjunction with its habitat term, as _Spartina-Elymus-poium_, _Picea-Pinus-hylium_, _Cereus-Yucca-eremium_, etc. Apparently a somewhat similar nomenclature is adapted to successions. The cause which produces a new habitat may well furnish the basis for the name of the general groups of successions, as _pyrium_ (literally, a place or a habitat burned over), a burn succession, _tribium_, an erosion succession, etc. A burn succession consists of a sequence of certain formations in one part of the world, and of a series of quite different ones, floristically, in another. A particular burn succession should be designated by using the names of a characteristic facies of the initial and ultimate stages in connection with the general term, e. g., _Bryum-Picea-pyrium_, etc. A trinomial constructed in this way represents the desirable mean between definition and brevity. Greater definiteness is possible only at the expense of brevity, while to shorten the name would entirely destroy its precision. The following classification of successions is proposed, based upon the plan outlined above. The termination _-ium_ (εῖον) has been used throughout in the construction of names for successions, largely for reasons of euphony. If it should become desirable to distinguish the names of formations and successions by the termination, the locative suffix _-on_ (-ών) should be used for the latter. The terms given below would then be _hypson_, _rhyson_, _hedon_, _sphyron_, _prochoson_, _pnoon_, _pagon_, _tribon_, _clyson_, _repon_, _olisthon_, _xerasion_, _theron_, _broton_, _pyron_,