With one or two exceptions all the genera are marine. They have a wide distribution at the present day, and are especially characteristic of far northern and extreme southern latitudes. The gigantic forms _Lessonia_, _Macrocystis_ and others already alluded to, belong to this group; also the genus _Sargassum_, of which the numberless floating plants constitute the characteristic vegetation of the Sargasso Sea. Palaeobotanical literature is full of descriptions of supposed fossil representatives of the brown algae, but only a few of the recorded species possess more than a very doubtful value; most of them are worthless as trustworthy botanical records. Many of the numerous impressions referred to as species of _Fucoides_ and other genera present a superficial resemblance to the thallus of the common Bladder-wrack and other brown seaweeds. Such similarity of form, however, in the case of flat and branched algal-like fossils is of no scientific value. In many instances the impressions are probably those of an alga, but they are of no botanical interest. The flat and forked type of thallus of _Fucus_, _Chondrus crispus_ (L.) and other members of the Phaeophyceae is met with also among the red and green algae, to say nothing of its occurrence in the group of thalloid Liverworts, or of the almost identical form of various members of the animal kingdom. The variety of form of the thallus in one species is well illustrated by the common _Chondrus crispus_ (L.). This alga was described by Turner[366] in his classic work on the _Fuci_ under the name of _Fucus crispus_ as “a marine Proteus.” It affords an interesting example of the different appearance presented by the same species under different conditions, and at the same time it furnishes another proof of the futility of relying on imperfectly preserved external features as taxonomic characters of primary importance. An example of a supposed Jurassic _Fucus_ is shown in fig. 49, and briefly described in the Chapter dealing with fossil Bryophytes. Several species of Flysch Algae have recently been referred by Rothpletz[367] to the Phaeophyceae under the provisional generic name _Phycopsis_, but they are of no special botanical interest. The extremely interesting genus _Nematophycus_ has lately been assigned by a Canadian author[368] to a position in the Phaeophyceae. Although the particular points on which he chiefly relies are not perhaps thoroughly established, there are certain considerations which lead us to include _Nematophycus_ as a doubtful member of the present group of algae. _Nematophycus._