8ir H. H. Eoivorth—The Western Baltic. 459 



been sophisticated in various ways, quite lately, in order to adapt 

 it to the conditions of brackish water in the Baltic. 



Let us now take another step. It will be quite obvious to 

 everybody that when the breakdown in the land-bridge so 

 frequently referred to in these papers took place it was not only 

 the Baltic and its fauna which was certain to be affected, but the 

 Cattegat ; only that in the latter case the effect would be the 

 opposite of what it was in the former. The breakdown in question 

 no doubt opened a gate for a great inflow of salt water from the 

 Cattegat, but the same operation opened a corresponding gate by 

 which the fresh waters of the vast Ancylus sea flowed into the 

 Cattegat, and in this way reduced their salinity very materially. 



Of this we have some very interesting and curious evidence in the 

 extinction of certain molluscs in the southern part of the Cattegat 

 which once lived and thrived there, as we know from the shell 

 mounds and raised beaches and kitchen middens of the northern 

 shores of the Danish islands and the southern shores of the Cattegat 

 itself. 



This important induction was first made in 1888 by the Danish 

 conchologist C. G. J. Petersen. He thus compares the fossil and the 

 living shells he had found in the Odense fiord in the island of Fuen 

 ("Om de Skalbserende Mollusken udbredningsforhold i de Danske 

 haven, etc.," p. 50) : — ^ 



Fossil Shells. Living Shells. 



Odostomia sp. TJtriculus obtusus. 



,, ,, Acera btdlata. 



■ Uiriculus frtmcatiilus. Hydrobia ulvce. 



Hydrobia ulvce, Sissoa membranacea. 



Rissoa membranacea. R. striata. 



JR. striata, var. Lacuna divaricata. 



R. inconspicua. Litorina litorea. 



Litorina litorea. ■ L. obtusata. 



L. rudis. L. rudis. 



L. obtusata. Cerithiimi retimlatmn. 



Cerithium reticulatwn. Nassa reticulata. 



Triforis perversa. Buccinum undatmn. 



Nassa reticulata. Tectura testudinalis. 



A.no7nia squamula. Modiolarea discors. 



Ostrea edulis. Cardium edule. 



Mytilus edulis. C. exigimm, var, 



Cardium exigmmi (typical). Mytilus edulis. 



C. edule. Macoma {Tellina) balthica. 



Macoma [Tellina) balthica. Abra alba. 



Sorobicularia plana. A. nitida. 



Venus (Tapes) pullastra. Solen pellucidus. 



V. (T.) aicreus. Mactra subtruncata. 



Montacuta bidentata. Thracia papyracea. 



Mya arenaria. Corbula gibba. 

 And iu a neighbouring place a valve Saxicava rugosa. 



of Tapes decussatus. Mya truncata. 



M. arenaria. 



Petersen was the first to see and to emphasize the importance of 

 these lists, especially when compared with other evidence to be 

 presently referred to. He pointed out that among the molluscs 



