338 HISTORICAL PALAZONTOLOGY. 
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 Aoraminifera, 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 Gren- 
eve 
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Fig 254.—Left valve of Pecten Islandicus. Glacial and Recent. 
landicus, Scalaria Grenlandica, Leda truncata, Astarte borealis, 
Tellina proxima, Natica clausa, &c. 
III. Post-Guiacrat Deposirs.—As the intense cold of the 
Glacial period became gradually mitigated, and temperate 
