148 



would prevail, to be followed by the more temperate types of 

 the Boreal and Celtic Provinces. With the retreat of the ice 

 beyond the point where the North Sea becomes on? with the 

 Atlantic, an influx of animals would then ensue from the north 

 which continues at the present time. 



According to the eminent French conchologist, M. Paul 

 Fischer*, the Straits of Dover are somewhat of a barrier to 

 a large number of shells peculiar to the Lusitanian Province, 

 or otherwise they would extend into the North Sea, and this 

 may perhaps explain the rarity of Lusitanian forms on the 

 Cleveland Coast. But if this barrier has been effective with 

 one group of shells, it seems difficult to understand why it 

 should not have been effective with the Celtic and Boreal 

 groups, as they spread northwards after the Ice Age. This 

 may have been so. but in that case the marine fauna must 

 to a great extent have entered the German Ocean from the 

 north, as just described. Movements of marine animals 

 through the Straits of Dover northwards, and from the 

 Atlantic southwards, must have taken place, though perhaps 

 on a smaller scale in the former than in the latter. Again, 

 these distributions would be interrupted by the conversion 

 of the sea floor into dry land, an event which certainly oc- 

 curred in post-glacial times. After the subsidence of the 

 land the sea would once more prevail and bring with it its 

 inhabitants. 



Thoroughly to trace the history of the marine fauna, we 

 should have to investigate the geographical distribution of 

 every species, and that of the genus to which it belongs. Next 

 we should have to ascertain the distribution of the genus in 

 time, and find out at what geological epoch, and in what 

 country it became most numerous in species. Only after we 

 had collected this vast body of facts, would we be in a position 

 to discuss the history of our local marine fauna. To do so 

 here is out of the question, but to illustrate this line of research 

 let us take the case of the genus of bivalve shells known as 

 Astarte, of which two species have been recorded from the 

 coast, A. sulcata and A. compressa. The first of these ranges 

 far north to Novaya Zemlia. the White Sea, and North America, 

 and extends southwards to the Canaries and throughout the 



* Manuel de Conchyliologie, p. 145. 



