sted as regards the other sea-fish: plaice, flounder, cod and sprat, 

 etc. Southern forms of molluscs, too, such as Tapes, Littorina, Ost- 

 rea, came with the Atlantic waters into the Baltic. Oyster-banks 

 existed everywhere in Isefjord, and around the Danish Islands. This 

 was the period of the »kitchen-middens »; the great fish-migration 

 period, when the gates of the Baltic stood wide open to the fish from 

 the Atlantic. All the species of fish found in the Baltic, except the eel, 



sea-pike, and mackerel, are to be considered as relicts from the Littorina 

 period, at the close of which began the rise of the land which restored 

 the depth-conditions of the Sund and the Belt to about their present 

 position. This geological alteration, which was completed about 

 3,000 years ago, had a far-reaching influence on the fish-races of the 

 Baltic, which are now in a greater or less degree separated from their 

 relations in the ocean and live und other hydrographical conditions, 



Surface-map (present time). 



Surface-map (Littorina-epoch). 



which have gradually altered their physiological life-conditions and 

 have even commenced to set their stamp on the bodily and exterior 

 form of the fish. 



The expression »relict» must, however, be understood relatively, 

 The ancient relicts from the days of the Polar Sea consist, at present, 

 of some lower animal forms, mostly inhabiting the great depths and 

 including too the arctic gray-seal, which still lives in the Baltic and 

 in lake Ladoga in Russia. These are relicts in the original sense of 

 the word, from a period dating 50,000 — 100,000 years back. Amongst 

 them there is only one certain arctic species of fish, the Cottus quad- 

 ricornis. 



Among the relicts of the Littorina period we reckon, in the first 

 place, the stromling or small herring, the indigenous herring race of 

 the central and northern Baltic, which now lives isolated from the 

 North Sea-herring, whose migrations nowadays do not extend past 

 the portals of the Baltic Sea. From the first, the stromling was a 

 local race — a relict of the great herring migrations of a couple of 

 thousand years ago, which has survived and gradually differentiated 

 physiologically and even morphologically into a new species. 



After the maximum of the Littorina epoch, which occurred 

 before the beginning of the Bronze-age, in the time of the »Kjok- 

 kenmoddinger » (ancient refuse-deposits) 4000 — 5000 years ago, an 

 upheaval of the landsurface occurred which lessened the depth of 

 the Oresund to something like the present. This elevation of the 

 land was almost complete at the close of the Bronze-age 600 — 500 

 b. C. From then, for the last 2500 years the bottom of the Oresund 

 has remained nearly constant, its elevation increasing only some 

 0.25 meter since that time. The atmospheric conditions also changed 

 into the cold climate of the early Iron-age. The transition from the 

 warm climate of the Bronze-age was according to Sernander accomplis- 

 hed in a few centuries, 650 — 400 b. G the last stage of the Bronze-age. 

 The temperature of this epoch (»the »Fimbul-winter » of the Sagas) 

 must have been considerably lower, for in the peat-layers from that 

 time we find deposits of subarctic forms. From the end of the Bronze- 

 age a gradual elevation of the land has been and is still in progress. 

 Sernander estimates the elevation for the last 2,000 years to: 



In the environments of Upsala 10 meter 



On the West-coast of Sweden 5 » 



In Oresund 0.24 » 



On the Bohuslan-shore are found sub-fossil remains of Ostrea 

 edulis, Tapes decussatus a. d. which formed the ancient oyster-beds 

 of the Littorina epoch, but which now are either extinct or survive 

 only as relicts in a few protected localities. They bear testimony 

 both to the climatic deterioration and to the upheavel of land during 

 and after the Bronze-age. 



In Greenland, Spitzbergen, Frans Josefs land and on the coast 

 of North America subfossil deposits are found of molluscs which 

 must once have lived in warmer waters. This proves that the land- 

 elevation and the deterioration of climate have passed over the entire 

 North Atlantic coast. I will give some examples in proof of this: 



The Swedish expeditions found in numerous places on Spits- 

 bergen sub-fossil deposits of Mytilus eclulis 1 )Among such subfossil 

 relicts from a more temperate sea found on Spitzbergen there are, 

 beside Mytilus, also Cyprina Islandica, Littorina littorea and Anomia 

 squamula, all of which can not live in the fjords of Spitzbergen under 

 the present conditions. 



On the shores of the southeast coast of the Disco-bight on Green- 

 land A. S. Jensen 2 has found fossil shells of Anomia squamula and 

 Zirphea crispata. The present limit of these molluscs is the south 

 coast of Labrador and the St. Lawrence bight, which shows that 

 during some part of the post-glacial time a warmer climate must have 

 prevailed than now. 



The chief representative of this warmer postglacial period in 

 the mollusc-fauna of East-Gronland and of Franz Josefs Land is 

 Mytilus edulis 3 . On Iceland it is represented by Purpura lapillus 4 

 a. o. 



It is probable that all these southern species of molluscs lived 

 simultaneously in the North Atlantic ocean and its ramifications 

 during this warm postglacial period. For the more distant parts 

 of the ocean this view still must be regarded as a hypothesis but in 

 Sweden and Denmark we have archseologic finds excavated from the 

 shore-deposits which enable us to discern between the warmer and 

 the colder period and to fix the time-limit with some approach to certi- 

 tude. The southern mollusc-species are found together with remains 

 from the Bronze-age. As to the next period, the rarity of archeeologic 

 finds in the graves from the early Iron-age about 400 b. C. to 100— 200 

 a. G. shows that the high stage of civilisation in the Bronze-age had for 

 some reason or other declined and thatthepopulationhad decreased and 



1 G. Andersson found Mytilus also on King Charies island about 40 M. above the sea-level; the Norwegian geologist Staxrud found the same mollusc at 60 M. on Spitzbergen. 

 V. Nordrnann: Anomia squamula som Kvartser-fossil paa Spitzbergen Meddel. Dansk Geol. Forening Bd 4 Kobenhavn 1912. 



2 Ad. S. Jensen: On the Mollusca of East Greenland I. With an introduction on Greenlands fossil Mollusc-fauna from the quaternary time. Medd. om Gronland, Bd 29. 

 1909 (Reprint 1905). 



Ad. S. Jensen and Poul Harder: Post-glacial changes of climate in arctic regions as revealed by investigations on marine deposits. Postglaziale Klima ver&nderungen seit 

 dem Maximum der letzten Eiszeit. Stockholm 1910. 



3 A. G. Nathorst. Bidrag till nordostra Gronlands geologi. Geol. Foreningens Forh. Bd. 23. Stockholm 1901. 



* G. Bardarson, Purpura lapillus i hsevede Liig paa Nord Kysten af Island. Vidensk. Meddel. Naturhist. Foren. i Kopenhavn 1906—1907. 



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