PHYSICAL CONDITIONS— POSTGLACIAL. 501 



Norway and Sweden. Many of the arctic forms which occur 

 in the older glacial clays are now wanting, while certain types 

 of a southern facies begin to appear. As the land emerged 

 the latter became more numerous, while at the same time 

 the boreal and arctic forms retreated. It is remarkable that 

 the southern molluscs were not only individually abundant, 

 but their shells were larger and better developed than those of 

 their descendants that still linger in greatly diminished numbers 

 in the adjacent seas. Evidently the conditions under which 

 they now live are less favourable than those that were expe- 

 rienced in postglacial times. Certain molluscs which were 

 formerly plentiful upon the south coast of Scandinavia no 

 longer occur there, but have retired to the more genial waters 

 of the west coast. The postglacial shell-beds of Spitzbergen 

 tell the same tale. The common mussel, at one time abundant 

 in the fiords of that region, has apparently become extinct, and 

 Cyprina islandica and Zittorina litorea, which are associated in 

 the Spitzbergen deposits with the mussel, have likewise dis- 

 appeared from those inhospitable shores. Again, Dr. Rink men- 

 tions that a number of shells which were obtained by him from 

 the clay-beds of Sarpiursak in Greenland were examined by 

 0. A. L. Mbrch, who found that they belonged partly to species 

 still existing on the coasts of North Greenland and partly to 

 more southern forms. Nor are we without similar indications 

 in the marine postglacial beds of Scotland of a formerly more 

 genial climate. Mr. Crosskey has drawn special attention to 

 the so-called " Peden-maximus bed " of the Clyde, which con- 

 tains shells (Psammobia ferroensis and Tellina incarnata), of 

 larger size and in greater numbers than they at present occur 

 living in the neighbouring sea. 



The recent dredging expeditions which have been sent out 

 from our own and other countries have moreover familiarised us 

 with the fact that Mediterranean forms are now and again 

 encountered in our northern seas, where they look strangely out 

 of place. Thus Sir Wyville Thomson mentions that in 110 

 fathoms, about 40 miles off Valentia, the dredge brought up a 



