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. 
Jou. 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, Anholt’s N. W. Reef 
and Gjedser Reef, which is the reason, why they are not overgrown with Algæ. 
The extent of the true stone-reefs, the surface of which consists only of stones, is 
relatively small; on the larger banks and flats 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 at 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 32/0, while in the true Baltic (east of Gjedser—Darsserort) it is or- 
dinarily less than 10°/0, 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. Jon. PETERSEN, Kanonbaaden Hauchs Togter, 1898, Kort III. 
* MARTIN KNUDSEN, Havets Naturlere. Hydrografi med særligt Hensyn til de danske Farvande. 
Skrifter udg. af Kommissionen for Havundersogelser. Nr. 2. København 1905. 
De internationale Havundersogelser 1902-1907. Skrifter udg. af Kom. f. Havundersog. Nr. 4. 1908. 
J. P. JACOBSEN, Mittelwerte von Temperatur und Salzgehalt, bearbeitet nach hydrographischen 
Beobachtungen in Dan. Gewässern 1880—1907. Meddel. fra Komm. for Havundersogelser Ser. Hydro- 
grafi. Bind I, Nr. 10. 1908. 
Nautical-meteorological Annual 1902—1906, published by the Danish Meteorological Institute. 
I am much obliged to Mr. J. P. Jacossen 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 hydrography of the Danish waters. 
