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SOME MINOR PROBLEMS CONCERNED IN THE 

 LOCAL DISTRIBUTION OF ANIMALS AND 

 PLANTS IN THE NEIGHBOURHOOD OF 

 FELSTEAD, ESSEX. 



r.y J. TRENCH. 



npHE distribution of animal forms in a great river which flows 

 through several degrees of latitude occurs under circumitances 

 of which we can more or less give an account. Climatal changes 

 there affect the distribution, both directly and indirectly. In small 

 streams, such as our Essex rivers, this disturbing element is elimi- 

 nated, yet ineciualities of distribution still prevail, and it is clear, 

 therefore, that some other explanation must be sought. 



Our first case of this inequality of distribution will be that of the 

 Fresh-water Molluscs. In the account of the Chelmer Expedition 

 (Essex Naturalist, vol. v., p. 253), among the list of aquatic 

 mollusca observed, is Planorbis coriieus. This shell is not now to 

 be found above Hartford End, either recent or fossil. We may, 

 perhaps, therefore regard it as a new introduction. In the same list 

 occurs Ncretina fluviatilis. If this shell now lives above Hartford 

 End it is exceedingly rare. I believe it to be extinct. As a fossil, in 

 the Alluvium it is common, and we may therefore suppose it to have 

 retreated down stream. Aficylus lacustris I have never taken in the 

 Chelmer above Hartford End, and I believe it to be rare, or quite 

 extinct, there, although it is frequent enough in the ponds about 

 Felstead. These instances are sufficient to show that there is a 

 difference in the distribution of the molluscs in the Chelmer as the 

 estuary of the river is approached. 



Some other cases of local distribution appear to have a bearing 

 on this question. The following table, compiled for the neighbour- 

 hood of Felstead, will illustrate cases : — 



