The term Miocene (from +meion+, meion, _less_, and +kainos+, cainos, _recent_) is intended to express a minor proportion of recent species (of testacea), the term Pliocene (from +pleion+, pleion, _more_, and +kainos+, cainos, _recent_) a comparative plurality of the same. It may assist the memory of students to remind them, that the _Mi_ocene contain a _mi_nor proportion, and _Pl_iocene a comparative _pl_urality of recent species; and that the greater number of recent species always implies the more modern origin of the strata. It has sometimes been objected to this nomenclature that certain species of infusoria found in the chalk are still existing, and, on the other hand, the Miocene and Older Pliocene deposits often contain the remains of mammalia, reptiles, and fish, exclusively of extinct species. But the reader must bear in mind that the terms Eocene, Miocene, and Pliocene were originally invented with reference purely to conchological data, and in that sense have always been and are still used by me. The distribution of the fossil species from which the results before mentioned were obtained in 1830 by M. Deshayes was as follows:-- In the formations of the Pliocene periods, older and newer 777 In the Miocene 1021 In the Eocene 1238 ---- 3036 ---- Since the year 1830 the progress of conchological science has been most rapid, and the number of living species obtained from different parts of the globe has been raised from about 5000 to more than 10,000. New fossil species have also been added to our collections in great abundance; and at the same time a more copious supply of individuals both of fossil and recent species, some of which were previously very rare, have been procured, affording more ample data for determining the specific character. Besides the reforms introduced in consequence of these new zoological facilities, other errors of a geological nature have been in many instances removed. POST-PLIOCENE FORMATIONS. I have adopted the term Post-Pliocene for those strata which are sometimes called post-tertiary or modern, and which are characterized by having all the imbedded fossil shells identical with species now living, whereas even the Newer Pliocene, or newest of the tertiary deposits above alluded to, contain always some small proportion of shells of extinct species.