26 



The above shell attains a size which is considerably 

 larger than any other British fresh-water shell. Local 

 causes, however, affect its dimensions and form very 

 considerably. In a cold stream near Chaigeley, in 

 Lancashire, it does not exceed two inches in length ; 

 whilst in a sheet of water at Hartwell, in Bucking- 

 Iiamshire, it attains a size which is above six and a 

 half inches. 



Amongst other causes which influence the form and 

 growth of the Anodonta may be enumerated, the deep- 

 ness, the purity, temperature, or rapidity of the water 

 in which it happens to be an inhabitant. 



The general form of our fresh-water muscle is that of 

 an elongated oval, being more or less ventricose. The 

 epidermis has a glossy appearance, and is somewhat of 

 a yellow tint, being slightly tinged with green. The 

 colour of the inner surface of the valves is usually 

 silvery, but occasionally pink, and, in rare cases, of a 

 somewhat bronze colour. 



This shell inhabits slow rivers, canals, pools, streams, 

 and lakes. 



There afe not only many varieties of form of the 

 Anodonta cygnea, but each variety sustains its charac- 

 ter under similar circumstances. Upon a careful ex- 

 amination of many specimens, I find no reason to differ 

 from the opinion of Professor Forbes, that we possess 

 but one Anodonta in Great Britain, and that the great 

 diversity of shape is to be attributed to local causes ; 

 for if we take two apparently quite distinct species of 

 this shell, we can find a dozen others which will 

 gradually assume in the first six specimens less and less 

 of the'character of the first shell, and in the last six 

 gradually more and more^of the second shell, by which 

 means we are compelled to acknowledge these two 

 seemingly diflereat shells to be merely two forms of the 

 same mollusc. 



