The Peridiniales play an important rôle in the Plankton flora of the sea and freshwater lakes, and have a world-wide distribution. In the narrative of the Challenger cruise they are described as occasionally filling the tow-nets with a yellow coloured slime[165]. Some genera, such as _Ceratium_, are found in enormous numbers off the British coast. As an example of the occurrence of fossil representatives of the Peridiniaceae reference may be made to one of two species of _Peridinium_ described by Ehrenberg in 1836. These were found in a siliceous rock described as Cretaceous in age from Delitzsch in Saxony. A comparison of Ehrenberg’s figures of the fossil species _Peridinium pyrophorum_ Ehrenb.[166], with those of the recent species _Peridinium divergens_ Ehrenb., as given by Schütt[167] and other writers, brings out clearly the very close resemblance if not identity of the two forms. Bütschli[168] in his account of the Dinoflagellata in Bronn’s _Thier-Reich_ confirms Ehrenberg’s determination of _Peridinium pyrophorum_, and points out its striking agreement with the recent species. II. COCCOSPHERES AND RHABDOSPHERES. (Organisms of doubtful affinity.) Our knowledge of these minute calcareous organisms is derived from Huxley’s description of coccoliths from the Atlantic in 1857, and from the accounts of Wallich, John Murray, and other writers. In the first volume of the narrative of the Challenger cruise[169] and in the volume on deep-sea deposits[170] these minute forms of life are figured and described. In the latter volume both genera are spoken of as extremely abundant in the surface waters of the tropical and temperate regions of the open ocean, and as forming an important constituent of the Globigerine ooze; they are said to occur entangled in the gelatinous substance of the Radiolarians, Diatoms, and Foraminifera, and are very common in the stomachs of Salps, Pteropods and other pelagic animals. Rhabdospheres are rare in regions where the temperature of the water sinks below 65° F.; the Coccospheres occur in tropical and temperate latitudes, and extend further north and south than the Rhabdospheres. As regards their botanical position, John Murray expresses the view that they are in all probability pelagic algae. In the interesting memoir by Schütt on the _Pflanzenleben der Hochsee_[171] there occurs a short reference to the forms described in the Challenger Reports, but they were not obtained by the staff of the Hensen Plankton Expedition and Schütt’s remarks are not based therefore on personal observations. While admitting the existence of such bodies, he points out that Zoologists have referred Coccospheres and Rhabdospheres to the algae as organisms which cannot be included in any group of animals, and Schütt is unable to recognise a sufficient reason for referring them to this class of plants. It is suggested indeed that they may be purely inorganic structures.