92 VARIATION IN UNIO AND ANODONTA chap. 



that the principal changes were as follows : the thickness, and 

 consequently the weight, of the shells became diminished, the 

 size of the beaks was reduced, the shell became highly colom-ed, 

 and diminished considerably in size, and the breadth of the shells 

 increased in proportion to their length (Fig. 36). Shells of 

 the same species of Cardmm, occm-ring in Lake Mareotis, were 

 found to exhibit very similar variations as regards colour, size, 

 shape, and thickness. 



Unio pictorum var. compressa occurs near Norwich at two 

 similar localities six or seven miles distant from one another, 

 under circumstances which tend to show that similar conditions 

 have produced similar results. The form occurs where the river, 

 by bending sharply in horse-shoe shape, causes the current to 

 rush across to the opposite side and form an eddy near the bank 

 on the outside of the bend. Just at the edge of the sharp 

 current next the eddy the shells are found, the peculiar form 

 being probably due to the current continually washing away the 

 soft particles of mud and compelling the shell to elongate itself 

 in order to keep partly buried at the bottom.^ 



The rivers Ouse and Foss, which unite just below York, are 

 rivers of strikingly different character, the Ouse being deep, 

 rapid, with a bare, stony bottom, and little vegetable growth, 

 and receiving a good deal of drainage, while the Foss is shallow, 

 slow, muddy, full of weeds and with very little drainage. In 

 the Foss, fine specimens of Anodonta anatina occur, lustrous, 

 with beautifully rayed shells. A few yards off, in the Ouse, the 

 same species of Anodonta is dull brown in colour, its interior 

 clouded, the beaks and epidermis often deeply eroded. Precisely 

 the same contrast is sliown in specimens of Unio tumidus, taken 

 from the same rivers, Ouse specimens being also slightly curved 

 in form. Just above Yearsley Lock in the Foss, Unio tumidus 

 occiurs, but always dwarfed and malformed, a result probably due 

 to the effect of rapidly running water upon a species accustomed 

 to live in still water.^ Simroth records the occurrence of remark- 

 ably distorted varieties in two species of Aetheria which lived in 

 swift falls of the PJver Congo.^ 



A variety of Limnaea peregra with a short spire and rather 

 strong, stoutly built shell occurs in Lakes Windermere, Derwent- 



^ J. 13. Bridgman, Quart. Jourii. Conch, i. p. 70. 

 - W. C. Hey, Journ. of Conch, iii. p. 268. ^ Zool. Anz. xiii. p. 662. 



