CHAP. I. RECENT, POST-PLIOCENE, AND POST-TERTIARY. 5 



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. 



Since the first introduction of the terms above defined, the 

 number of new living species of shells 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 species 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 proportion of recent to extinct species with the 

 definitions first given by me, or which are implied in the 

 etymology of those terms. These innovations have been 

 treated of in my " Elements or Manual of Elementary 

 Geology," and in the Supplement to the fifth edition of the 

 same, published in 1859, where some inodifications of my 

 classification, as first proposed, are introduced; but I need 

 not dwell on these on the present occasion, as the only 

 formations with which we shall be concerned in the pre- 

 sent volume are those of the most modern date, or the 

 Post-tertiary. It will be convenient to divide these into two 

 groups, the Recent and the Post-pliocene. In the Recent we 

 may comprehend those deposits in which not only all the 

 shells but all the fossil mammalia are of living species; in the 

 Post-pliocene those strata in which, the shells being recent, 

 a portion, and often a considerable one, of the accompanying 



