Mr. W. Bateson has made[195] some observations on the shells of _Cardium edule_ taken from a series of terraces on the border of certain salt lakes which once formed a portion of the Sea of Aral. As these lakes gradually became dry, the water they contained became salter, and thus the successive layers of dead shells deposited on their borders form an interesting record of the progressive variation of this species under conditions which, in one respect at least, can be clearly appreciated. At the same time the diminishing volume of water, and the increasing average temperature, would not be without their effect. It was 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 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.[196] 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 accustomed to live in still water.[197] Simroth records the occurrence of remarkably distorted varieties in two species of _Aetheria_ which lived in swift falls of the River Congo.[198]