348 MARINE BIOTIC COMMUNITIES 



the terms applied to communities of different rank in this discussion 

 are inserted after those used by the author quoted. The variation 

 appears rather small to fit our concept of the association ; the brackets 

 indicate the nomenclature of our system. 



"1. A Nucula-Corbula-association [faciation], characterized too 

 by its nearly total want of Solen. This association [faciation] is 

 found purest in Nussum Bredning, where the Nucula amount for a 

 series of years has been ca. 30-50 g. per square meter; during the 

 last years it has decreased a little, yet only to ca. 10 g. In the other 

 Brednings, an amount of Nucula exceeding 10 g. only appears as an 

 exception. Corbula, which is found in great amounts in Nissum 

 Bredning together with Nucula, is probably in much smaller degree a 

 characteristic animal; it is, for instance, still found in rather important 

 amounts in Skive Fjord. In the Nucula-Corbula association [facia- 

 tion] , Abra is only found in smaller amounts, Solen is rare and Alya 

 truncata is not found at all. That Abra and Solen appear in such 

 small amounts in Nissum Bredning is probably caused by the fact 

 that they are specially persecuted by the plaices. 



"2. An Abra-Solon association [faciation], where Nucula and 

 generally also Corbula are of subordinate importance. This associa- 

 tion [faciation] is most typically found in Liv0 Bredning. As men- 

 tioned above, the two bivalves are found in very fluctuating amounts 

 in the various years. 



"In Lavbjerg and Kaas Brednings transitions between the Nucula- 

 Corbula and Abra-Solen associations [faciations] are found. 



"3. An Abra-Solen-Mya {truncata) association [ecotone], found in 

 the side-Brednings originating from Liv0-L0gstr0 Bredning, respec- 

 tively Thisted-Visby Brednings and Risgaards and Lovns Brednings. 

 This association is displaced by the occurrence of Mya truncata. 



"When entering into shallow water or in the very inmost Bred- 

 nings, this association is displaced by the Macoma baltica formation 

 [biome]. One of the characteristic animals of this formation, Mya 

 arenaria, is found together with animals of the Abra-Solen-Mt/a trun- 

 cata association [faciation] in Lovns Bredning as mentioned above." 



Another important type of variation has been discovered in the 

 Echinocardium-Thyasira community by Davis. On the Dogger Bank, 

 he described several areas dominated by Spisula [Mactra) subtrun- 

 cata (da Costa), from 500 billion to 5 trillion individuals covering 

 several hundred square miles. He believes that these result from fail- 

 ure of the spat to scatter about (Fig. 80, p. 343). 



Changes from year to year (annuations) are shown in Fig. 40. 

 The near absence of some of the dominants in certain years is notice- 



