550 Sir H. H. Hoivorth — Geological History of the Baltic, 



IV. — The Eecent Geological History of the Baltic. 



Pabt IV: Climatic and othek issues. 



By Sii- H. H. Howorth, F.R.S., F.G.S. 



IN the first part of this series, p. 313, I referred to the recent 

 invasion of the Eastern Baltic by the Mya arenaria, as first 

 pointed out by Dr. Nathorst. Dr. Petersen has called my attention 

 to an important paper by A. S. Jensen, which has apparently been 

 overlooked by English conchologists, and I propose to condense his 

 results, as they are very noteworthy from their geological lessons. 



The shell has hitherto been treated as a typical Arctic shell. Thus 

 Crosse and Debeaux write of its original home : " De I'ocean Glacial 

 arctique, qui parait etre sa veritable patrie, elle est descendue dans les 

 mers du nord de I'Europe, jusques et y compris la Manche et une partie 

 de nos cotes de I'Ocean " (Journ. de Conchologie, ser. in, iii, 254, 

 1863). Gwyn Jeffreys, in describing the shell, says, " The occurrence 

 of this circumpolar shell-fish so near the tropic of Cancer probably 

 indicates the most southern limit in space of the glacial epoch " (Brit. 

 Conch., iii, G5, 66). 



Jensen has most clearly shown that these inferences are quite 

 unjustifiable, and that the shell has, in fact, no pretension to be an 

 Arctic shell at all. He has proved that in every case where it has 

 been reported from Greenland, Iceland, Spitzbergen, Nova Zembla, 

 the river Yenissei, and Bering's Strait, it has been mistaken for 

 Mya truncata, var. ovata, and that its true habitat is limited to the 

 North Temperate Zone. It extends southwards in the Atlantic as far 

 as South-West France in Europe, and South Carolina in America, and 

 in the Pacific to Japan and China between lat. 30° and 40° N. lu 

 the European area it is found in the British seas, Belgium, Holland, 

 Germany, Denmark, and Southern Sweden. Sars reports it from 

 the whole Norwegian coast, while Knipowitsch does so from the 

 warm section of the "White Sea (see Jensen, " Studier over nordiske 

 Mollusker," part i, Mya. Videns. meddel. Naturhist. for. i Kjoben- 

 havn, p. 150, 1900). 



This is not all. The curious fact remains that while its dis- 

 tribution is as here mentioned, and while it is virtually certain that 

 it has migrated, not from the north, but from the south or west, it 

 should have entered, not only the Eastern Baltic, but apparently the 

 Scandinavian waters generally, so recently. It is still a most 

 valuable article of food, yet it has never occurred in the Danish 

 kitchen middens, where so many other edible molluscs occur. Nor 

 has it been found in the raised beaches or the deposits in the Isefiord, 

 etc., in the southern part of the Cattegat, now known as the Tapes 

 beds, which synchronize with the kitchen middens, so that, as 

 Petersen argued, it doubtless invaded the Baltic waters in the 

 recent human period. 



Brogger has discussed its distribution on the Norway coasts, and 

 argued forcibly that there also it is a recent arrival, since it does not 

 occur in the raised beaches properly so called. In the Christiania 



