With respect to the distribution of the Foraminifera according to geological periods, we may briefly state that they have been found in every formation from the Silurian to the Tertiary. The species, at first very simple in their forms, begin to appear in increasing numbers in the carboniferous formations. They become more numerous, and, at the same time, more complex in their forms, in the Cretaceous period; they are still more diversified, and appear to have multiplied much more rapidly in the Tertiary period, where they attain the maximum of their numerical development. In the celebrated quarries of St. Peter, at Maestrecht, the _Siderolites calcitrapoides_ of Lamarck are found in the upper chalk (Fig. 23). In the calcareous formation of Chaussy, in the Seine and Oise district, and other parts of the Paris basin, the _Fabularia discolithes_ (Fig. 24) of Defrance is found. Finally, the _Dactylopora cylindracea_ of Lamarck (Fig. 26) is found in the eocene formation of Valmondois and in the chalk of Grignon. At first, this little creature was thought to be a polype; but d'Orbigny, in his "Prodrome de Paleontologie," has placed it among the Foraminifera, thinking that it appeared to occupy a place between the two classes. [Illustration: Fig. 24. Fabularia discolithes (Defrance). Natural size and magnified.] [Illustration: Fig. 25. Alveolina oblonga (d'Orbigny). Natural size and magnified.] [Illustration: Fig. 26. Dactylopora cylindracea (Lamarck). Natural size and magnified.] The existing Foraminifera are by no means equally distributed in every ocean. Some genera belong to warm countries, others to temperate and cold climates. They are much more numerous, however, and much more varied in their forms, in warm than in cold climates, and, we may add, larger also, for Sir E. Belcher brought a recent species from Borneo which measured two inches in diameter. * * * * *