12 



The nature of the bottom. The most important kinds occurring in the 

 Danish waters, are (1) stony bottom, (2) sand-bottom and gravel-bottom, (3) mixed 

 bottom consisting of a mixture of sand and clay or mud and (4) soft bottom con- 

 sisting of clay or mud. To these may be added (5) rocky bottom and (6) compact 

 clay (tertiary or glacial). I shall not endeavour here to describe more closely the 

 distribution of these kinds of bottom, as the nature of the bottom is very variable 

 from place to place. With regard to the Kattegat reference may be made to C. G. 

 JoH. Petersen's chart'. For the rest some information is given below in the list 

 of my dredgings; it may however be remarked, that I have chiefly dredged at 

 places with stony bottom. The rule is, that sand-bottom is connected with shal- 

 lower water, soft bottom with the deeper and mixed bottom with the intermediate 

 depths. The stones are mostly found in shallow water and on reefs, which are 

 for a great part noted on the charts. There are grounds, however, the surface of 

 which is exclusively or predominantly sand , e. g. Horns Reef, Anholl's N. W. Reef 

 and Gjedser Reef, which is the reason, why they are not overgrown with Algae. 

 The extent of the true stone-reefs, the surface of which consists only of stones, is 

 relatively small; on the larger banks and Hats stony bottom is ordinarily inter- 

 mixed with gravel, sand or even clay. In deep channels with strong current stony 

 bottom is often found, which is kept clean by the current. Rocky bottom is found 

 at several places near Bornholm , but elsewhere is scarcely known with certainty; 

 it occurs perhaps at some places in the Skagerak near Hanstholm and Bulbjerg. 

 On the other hand, firm glacial clay occurs al many places in the Skagerak and 

 firm tertiary clay at all events in the Little Belt. 



The salinity and temperature of the sea -water As these conditions 

 are of the greatest importance in understanding the distribution of the species, the 

 conditions which are of special importance for our subject may briefly be dis- 

 cussed here; for the rest, reference may be made to the hydrographical works 

 mentioned below -'. In consequence of the fact that the salinity in the North Sea 

 is more than 32" o(), while in the true Baltic (east of Gjedser— Darsserort) it is or- 

 dinarily less than 10"miii, the greater part of the Danish waters is a mixed region 

 with complicated and variable hydrographical conditions, the most important mo- 

 ment in which is that the heavy North Sea water from the Skagerak penetrates 

 along the bottom through the deep channel in the eastern Kattegat and further as 



' C. G. JoH. Petehsen, Kanoiibaaden Hauclis Togter, 1893, Kort III. 



' Martin Knudsen, Havets Natiirlsere. Hydrografi med sterligt Hensyii til de danske Farvandc. 

 Skritter udg. af Konimissionen for Havundersogelser. Nr. 2. Kobenliavn 1905. 



De internationale Havundersogelser 1902- 1907. Skrifter udg. af Kom. f. Havundersag. Nr. 4. 1908. 



.1. P. Jacobsen, Mittelwerte von Temperatur und Salzgehalt, hearbeitet nacb hydrographisclien 

 Beobaclitungen in Dan. Gewassern 1880 — 1907. Meddel. fra Komm. for Havundersagelser Ser. Hydro- 

 grafi. Bind I, Nr. 10. 1908. 



Nautical-meteorological Annual 1902 — 1906, published b\' the Danish Meteorological Institute. 



I am much obliged to Mr. J. P. .Jacohsen for placing at my disposal some unpublished lists 

 with hydrographical averages. I am much indebted to Mr. Martin Knudsen and Mr. J. P. Jacobsen 

 for various pieces of information regarding the hydrograph}' of the Danish waters. 



