92 VARIATION IN UNIO AND ANODONTA chap. 



found 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 

 coloured, 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 Cardium^ occurring in Lake Mare- 

 otis, 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 cur- 

 rent 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 shown 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 

 occurs, but always dwarfed and malformed, a result probably 

 due to the effect of rapidly running water upon a species accus- 

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

 remarkably distorted varieties in two species of Aetheria which 

 lived in swift falls of the River Congo.'^ 



A variety of Limnaea peregra with a short spire and rather 

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



1 J. B. Bridgman, Quart. Journ. Conch, i. p. 70. 



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



