Sir H. H. HoivortJi — Geological History of the Baltic. 399 



island of Laeso, which, as I showed in a former paper, is surrounded 

 by a shallow sea bottom and is covered with the debris of a 

 littoral fauna; and Avhich it is generally thought was at a not 

 distant date joined to the mainland of Jutland. At all events it is 

 plain that we have in tliis southern section of the Cattegat a very 

 distinct marine sub-province which ought to be united, not with the 

 northern section, but with the Sound, and is marked notably by the 

 presence of a considerable number of shells which are absent in 

 the northern section. 

 . The following shells are absentees there : — 



Gasteeopoda. 9. Troplion truncatus. 



1. Aclis ascarus. If*- Fusus (Neptunea) antiquiis. 

 Petersen, however, thinks the H- Scutellina fulva, vulgar. 

 dead shells so named found by Cattegat. 



Collin at Hellebach may be Aclis 12- Chiton alhus. 



supraniiida, which occurs in both Apparently only m the S. of 



hsts, see pp. 71 and 80. Cattegat Sound. 



2. Parthenia spiralis. 



Also found in the Limfiord, ib., Lamellibranchia. 



75. 1. Mijtilus ph«,seolimis. 



3. P. interstincta. A new shell in S. of the Cattegat 

 Also found in the Limfiord, ib. only, and not found N. of Laeso. 



4. Odostomia acuta. 2. Cardium fasciatum. 

 Subfossil in the Virk Sound. Not found N. of Laeso. 



5. O. unidentata. 3. Astarte borealis. 



Also fossil in the Limfiord. In the W. Baltic and S. Cattegat ; 



6. Triforis perversa. dead specimens W. of Laeso. 

 S.E. and S.W. of Laeso. 4. A. compressa or sulcata. 



7. Natica islandica. 5. Cyprina islandica. 



8. Vehttina Icevigata. 



The presence of those shells in the south, but not in the north, of 

 the Cattegat I would explain as probably due to the erratic history 

 of the great Danish gulf known as the Limfjord, which virtually 

 separates Jutland from Wendsyssel and which discharges itself into 

 the Southern Cattegat, of which it forms a kind of gulf. 



Through the Limfjord the Cattegat has had an intermittent com- 

 munication with the North Sea. Its eastern opening into the Cattegat 

 has always been open, but its western one into the North Sea has at 

 times been for a considerable period silted up and closed by a 

 cul-de-sac, as I mentioned in a previous paper (Geol. Mag., Dec. V, 

 Vol. II, p. 11), The continual breaking down in the eighteenth and 

 nineteenth centuries of the narrow isthmus separating the Limfjord 

 from the North Sea occasionally flooded its western part with salt 

 water from tlie latter sea, thus raising its salinity to 18 per thousand. 

 This led to the importation there of a considerable number of North 

 Sea fish and of certain molluscs like the oyster, Tapes pullastra, and 

 the typical form of Cardium exicfuum, which does not live in the 

 eastern part of the fjord. 



Morlot says tlie Canal of Agger by which the Limfjord entered the 

 North Sea had become so narrow that only small vessels could pass, 

 and it threatened to close altogether in 1859. 



It is not impossible that a number of the shells occurring in the 

 Southern Cattegat and not in the Northern may have entered it from 



