PECULIARITIES OF PLAICE FKOM DIFFERENT FISHING GROUNDS. 337 



In these specimens of glacialis, therefore, spinulation is undoubtedly 

 more developed than in the most spinulated plaice, even than in those 

 of the Baltic. On the other hand, the descriptions of the American 

 naturalists do not tend to show that the forms named by them 

 Lioindta Putnami and gkibra are more spinulated than the plaice 

 of the Baltic. It is clear, however, that all the local forms of glacialis 

 differ very distinctly from the plaice in the smaller number of fin-rays 

 and the character of the post-ocular ridge, which is granulated, termina- 

 ting posteriorly in a pear-shaped elevation of the skull-bone, succeeded 

 by a slight elevation of the post-temporal bone. These two elevations 

 correspond to the two posterior tubercles of the plaice, but are not 

 prominent enough to be called tubercles. 



We have seen that in the White Sea both plaice and glacialis occur ; 

 but this is the limit of the plaice eastwards and of glacialis westwards. 

 The two species similarly succeed one another on the coast of Alaska, 

 the northern limit of the plaice being the northern shore of the Alaskan 

 Peninsula, which also forms the southern limit of glacialis. Glacialis 

 therefore is strictly a geographical representative of the plaice. So far 

 as we know it is, like the plaice, a marine form, not ascending rivers 

 higher than their mouths. There are remarkable and interesting 

 differences in the limits between the two species in different parts 

 of the world, which are found on examination to correspond very 

 closely to differences of temperature depending on ocean currents. In 

 a map of the world by John Bartholomew the seas closed by ice 

 in winter in the north are distinguished, and the distribution of the 

 species glacialis corresponds almost exactly to the area of these seas. 

 Owing to the north-eastern trend of the coast of Scandinavia the Gulf 

 Stream, or north-easterly warm current in the Atlantic, travels far 

 to the north and east, producing an ice-free sea as far as the entrance of 

 the White Sea. Thus the plaice extends on the east side of the 

 Atlantic beyond the North Cape, latitude about 72°, while on the east 

 side of the Pacific its northern limit is about 56°. The westward 

 projection of Alaska stops the north-easterly progress of the warm 

 current in the Pacific, so that Behring Sea is closed by ice in winter, 

 and here we have the form glacialis. Again, the open warmer sea 

 in the Atlantic embraces the south coast of Iceland, where the plaice 

 is not only abundant, but reaches its maximum size ; while the glacial 

 sea extends along the coasts of Greenland, down the west coast of 

 America to Nova Scotia. The southerly cold current, known as the 

 Labrador current, passes down from the north along the coast of 

 Labrador and the east coast of North America, and this fact corresponds 

 to the southerly extension of the glacialis form to Cape Cod, and 

 the entire absence of the plaice. It appears that glacialis is taken 



