JORDAN: ON THE GENUS FUSWS. 227 



show that a temperature limit can be drawn within which the 

 British fauna may be restricted." 



The question at once arises : " What isotherm shall be 

 adopted ?" The answers given to this question would probably 

 reveal the greatest differences of opinion owing to the varying 

 temperature of the sea bottom near the British Isles. 



Off the north coast of Scotland in the Faroe Channel 

 there is probably a greater range of temperature of the sea 

 bottom than can be found in a similar area in any other part of 

 the globe. 



At one station the reading was as low as 29*2° Fahr. — 

 nearly three degrees below the freezing point of fresh water. 

 A few miles away at the same depth, 540 fathoms, the tempera- 

 ture was 46 "0° Fahr., a difference of nearly seventeen degrees, 

 and about midway between these stations at a depth of 300 

 fathoms the bottom temperature is 48*0° Fahr. It may be 

 mentioned that the temperature of 29-2° is lower than any 

 reading obtained in the " Valorous"" expedition at higher 

 latitudes in the Davis Strait and at double the depth. When 

 we find bottom temperatures varying 19° Fahr. in a few miles, 

 what shall be our guide in flying a temperature limit ? I feel 

 great diffidence in dissenting from the opinion of so great an 

 authority as Dr. Murray, but, nevertheless, I can see no reason 

 for selecting any particular temperature as a limit. If a tempera- 

 ture limit of 33 '4° Fahr. were adopted it would carry us across 

 the North Atlantic and a long distance up the Davis Strait. 



If 36 '5° be taken it would include the whole of the eastern 

 portion of the North Atlantic. If the isotherm of 4o"o'-' Fahr. 

 were adopted it would carry us from off the Faroe Banks south- 

 ward, at the 900 fathoms line, past Ireland to the west coast of 

 Africa, and the comparatively high bottom temperature of 47*^ 

 Fahr. is found upwards of 200 miles north-west of the Butt of 

 Lewis, at a depth of 570 fathoms, and about 90 miles off the 

 west coast of Ireland at a depth of 422 fathoms, and where shall 

 we stop? The fact is that the bottom temperatures which are 



