THE QUATERNARY PERIOD. 351 



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 characterize the Glacial beds 

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

 landicus. Scalaria Grccnlandica, Leda truncata, Astarte borealis, 

 Tcllina proximo, Natica claitsa. &c. 



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



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

 Glacial period became gradually mitigated, and temperate 

 conditions of the climate were once more re-established, various 

 deposits were formed in the northern hemisphere, which are 

 found to contain the remains of extinct Mammals, and which, 

 therefore, are clearly of Post-Pliocene age. To these deposits 

 the general name of Post-GIacial formations is given ; but it is 

 obvious that, from the nature of the case, and with our present 

 limited knowledge, we cannot draw a rigid line of demarcation 

 between the deposits formed toward the close of the Glacial 

 period, or during warm " Interglacial " periods, and those laid 

 down after the ice had fairly disappeared. Indeed it is ex- 

 tremely improbable that any such rigid line of demarcation 



