161 



The Estuarine Foraminifera of the Port 

 Adelaide River. 



By Walter Howchin, F.G.S. 

 [Read August 5, 1890.] 



Many of the Foraminifera have a cosmopolitan distribution^ 

 and are equally unrestricted with regard to depth. Not a few, 

 however, and particularly those of a high testaceous development, 

 are restricted to particular areas. The chief factors that are 

 operative in limiting the range of certain species are apparently 

 temperature and varying bathymetrical conditions. The restric- 

 tive influence of climate on many species offers an absolute 

 barrier to their extended migrations ; and whilst the limitations^ 

 as to depth are not so definite as those dependent on surface-tem- 

 perature, yet the majority of species are influenced by these con- 

 ditions. We have, therefore, to recognise a vertical as well as a 

 geographical distribution, and, generally speaking, there are 

 moderately distinct zones which carry respectively a f oraminif eral 

 fauna that is characteristic of the depths in which they are found.. 



Amongst the shallow-water species, there are some which 

 have become so far modified in relation to habitat as to flourish 

 under brackish water conditions, and in some instances, as, for- 

 example, Trocliamimna injlata, Miliolinafusca and Polystomella 

 striato-jninctata, even showing a preference for such positions. 

 The number of shallow-water Foraminifera, which may succeed in 

 establishing themselves in an estuary, will probably depend on the 

 relative sal tn ess of the water, and this, of course, will be regu- 

 lated by the variable proportions of sea- and surface-waters that 

 intermingle in different estuaries. The amount of fresh water 

 that may find its way into the Port River is no doubt propor- 

 tionately less than occurs with estuaries where the rain is more 

 continuous and the drainage from the land more direct than is 

 the case with the Port River. The latter is, indeed, more of an 

 inlet from the sea than an outlet of drainage from the land. 

 From this cause the condition of the harbour is probably more 

 distinctly marine than is usual with estuaries, which may account 

 for the comparatively large number of species obtained within 

 the area and greater robustness of shell-structure than is usual 

 with estuarine examples. 



The localities dredged were — 1. The main passage of the North 

 Arm, in the situations of (a) mid-stream, (h) on ground covered- 



