116 FOUKFOLD DIVISION OF TERTIAEY FORMATIONS. [On. X. 



this period contain an extremely small proportion of living species, which 

 may be looked upon as indicating the dawn of the existing state of tha 

 testaceous fauna, no recent species having been detected in the older or 

 secondary rocks. 



The term Miocene (from (xsjov, meion, less, and xaivo.c, cainos, recent) 

 is intended to express a minor proportion of recent species (of testacea), 

 the term Pliocene (from -TrXsiov, pleion, more, and xaivog, cainos, recent) a 

 comparative plurality of the same. It may assist the memory of stu- 

 dents to remind them, that the MiocQue contain a mmor proportion, and 

 Pliocene a comparative plnrsMij 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 spe- 

 cies 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 men- 

 tioned were obtained in 1830 by M. Deshayes was as follows : — 



In the formations of the Pliocene periods, older and newer - V'Z'r 

 In the Miocene ...... 102I 



In the Eocene .--.,. 123s 



3036 



Since the year 1830, the number of new living species obtained 

 from different parts of the globe has been exceedingly great, supplying 

 fresh data for comparison, and enabling the paleontologist to correct 

 many erroneous identifications of fossil and recent forms. New spe- 

 cies also have been collected in abundance from tertiary formations of 

 every age, while newly discovered groups of strata have filled up gaps 

 in the previously known series. Hence modifications and reforms have 

 been called for in the classification first proposed. The Eocene, Miocene, 

 and Pliocene periods have been made to comprehend certain sets of 

 strata of which the fossils do not always conform strictly in the propor- 

 tion of recent to extinct species with the definitions first given by me, or 

 which are implied in the etymology of those terms. Of these and other 

 innovations I shall treat more fully in the 14th and 15th chapters. 



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 



