208 BRITISH AND EUROPEAN FOSSIL ELEPHANTS. 



large rivers flowing frorn the south during the Pliocene 

 period, along the course of which the Hippopotami could have 

 migrated during winter, the argument might apply to the 

 population of one or two river-valleys, but it would hardly 

 extend to the Hippopotami spread over the broad area of 

 England. In balancing these various considerations, it seems 

 to me most probable that the Hippopotamus major was a per- 

 manent resident of the country during the Pliocene period. 

 This would involve a comparatively warm temperature 

 throughout the year as late as the deposition of the ' Grays 

 Thurrock ' beds, and the same would seem to be indicated by 

 the presence of some southern freshwater shells, which are 

 now extinct in England. 



It is at the present time very generally accepted among 

 geologists that there are good grounds for believing in a con- 

 tinual refrigeration of climate during the Pliocene period in 

 Britain. But this can only be entertained as holding good 

 in regard of the sea. The terrestrial evidence furnished by 

 the general characters of the mammalian fauna gives no 

 countenance to a corresponding reduction of temperature on 

 the land. It may be possible to reconcile the opposed indi- 

 cations by supposing that while the removal of an ocean 

 barrier in the form of islands to the north admitted the 

 gradual migration of Arctic marine forms, and repelled 

 Mediterranean species to more southern localities, still the 

 climatal conditions of the land may not have been altered in 

 the same proportion. One cause of this may be sought for 

 in the unbroken continuity of land between England and the 

 Continent during the Pliocene period, which would have 

 contributed to counterbalance the effects produced by the 

 decreasing temperature of the sea. 



I shall now briefly review the opinions of some English geolo- 

 gists who have been more especially engaged in researches upon 

 the newer Tertiaries. Mr. Searles Wood, in the introduction to 

 his admirable monograph of the Crag Mollusca, after giving 

 the division, at the time generally prevalent, of three dif- 

 ferent periods in the formation, namely, the Miocene coralline 

 crag, Pliocene red crag, and Pleistocene mammaliferous crag, 

 states his opinion that, upon the shell evidence, the lacustrine 

 or fluviatile deposit of Grays, Clacton, and Sutton, were pro- 

 bably the freshwater equivalents of the Red Crag, while the 

 Copford deposit may be of a more modern date. 



Mr. Prestwich, in his note on the gravel near Maidenhead, 

 after describing the lithological characters and range of the 

 mass, refers to the Brentford bed, and to the fossil bones dis- 

 covered there by Mr. W. Kirby Trimmer, in 1813, as pertain- 

 ing to it. With regard to the age of the gravel, he states 





