252 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



Common Pipe-fish ; Si2^honosfoma typlih, the Broad-nosed Pipe-fish ; and 

 still rarer, Zahrns mixtus, the Striped Wrasse. To these may be added 

 Mugil chflo, the Thick-lipped Grey IMullet, which haunts the shore, but 

 is an active wandering fish, not restricted in its movements like the 

 others. 



Heincke does not mention Lahrus maculaius, the Spotted Wrasse, 

 which, having a much more extended range than Lahnis mixtus, is more 

 likely to occur at Heligoland than the latter. Possibly this is a mis- 

 take, and maculatus should be substituted for mixtus in the above list. 

 Lahrus maculatus has certainly been taken at Yarmouth and Lowestoft, 

 and occurs all along the east coast of Britain, while mixtus has scarcely 

 ever been taken there. 



It is not likely that any of these species, except the grey mullet, 

 make long journeys at different seasons ; they are in all probability 

 resident where they are found in the region considered. They are 

 southern species, which are able to bear the winter cold : Heincke 

 states that they are driven in the cold months of the year from the 

 inter-tidal zone into somewhat deeper water. 



My conclusions concerning southern species entering the North Sea 

 from the south, are as follows : — 



(1) The area in which the more characteristic southern species, such 

 as mackerel and Trigla hirundo are found, is bounded by a line drawn 

 from the north coast of Norfolk in a north-easterly direction to the 20 

 fathom line, and following the latter limit to the Horn Reef. 



(2) Certain southern aperticolous species visit this area only in 

 summer, from May to October, and certain fundicolous species are found 

 there at the same time, but how far the latter are absent in winter is 

 not known. 



(3) The immigration of these southern forms at this period of the 

 year appears to be determined by the higher temperature due to the 

 season, not by an inflow of water taking place only at that season. The 

 uniform shallowness of the water is, however, an important factor, on 

 the one hand causing a great difference between summer and winter 

 temperatures, and, on the other, protecting the area in summer from the 

 influence of the cold water in the deeper part of the North Sea, to the 

 north and west. 



Southern Species Entering at the North. 



We next proceed to study the distribution and migrations of southern 

 species in the northern part of the North Sea. In a previous part of 

 this paper, I have already given a list of 18 species of southern forms, 

 which according to Mobius and Heincke are occasionally taken in the 



