338 HISTORICAL PALEONTOLOGY. 



deposits," with the bones of extinct Mammals, truly belong to 

 the Glacial period, being formed during warm intervals when 

 the severity of the Arctic cold had become relaxed. It is 

 further believed that some, at any rate, of the so-called " high- 

 level " river-gravels and " brick-earths " have likewise been 

 deposited during mild or warm intervals in the great age of 

 ice ; and in two or three instances this has apparently been 

 demonstrated deposits of this nature, with the bones of ex- 

 tinct animals and the implements of man, having been shown 

 to be overlaid by true Boulder-clay. 



The fossils of the undoubted Glacial deposits are principally 

 shells, which are found in great numbers in certain localities, 

 sometimes with Foraminifera, the bivalved cases of Ostracode 

 Crustaceans, &c. Whilst some of the shells of the " Drift" 

 are such as now live in the seas of temperate regions, others, 

 as previously remarked, are such as are now only known to 

 live in the seas of high latitudes ; and these therefore afford 

 unquestionable evidence of cold conditions. Amongst these 

 Arctic forms of shells which characterise the Glacial beds 

 may be mentioned Pecten Islandicus (fig. 254), Pecten Groen- 



Fig. 254. Left valve of Pecten Islandicus. Glacial and Recent. 



la?idicus, Scalaria Grcenlandica, Leda truncata, Astarte borealis, 

 Tellina proximo,, Natica clausa, &c. 



III. POST-GLACIAL DEPOSITS. As the intense cold of the 

 Glacial period became gradually mitigated, and temperate 



