VARIATION^ IN THICKNESS. 
79 
surface of what is or were the external shell-walls, and therefore 
preserving the same relative thicknesses of the external and internal 
shell-walls that distinguishes normal specimens. 
Fluviatile species, though affected by the comparative abundance 
or scarcity of calcic matter in relation to the thickness attained by 
the shell, are also influenced greatly by other circumstances of their 
environment, those individuals inhabiting rough or disturbed waters, 
rapid and turbulent streams, etc., develop a thicker and stronger 
shell than usual to better enable them to withstand the force of tlie 
waves and currents to which they are exposed, and often show a 
shorter spire and a more expanded and larger mouth, which necessarily 
allows for greater clinging or adhesive power, and renders the mollusk 
less liable to be detached and probably injured by wave violence. 
In Anodons the light inflated form with thinly calcified valves 
may, as in the Univalves, exhibit according to circumstances an 
actual scarcity of shell-forming material or a idiysiological inability 
to utilize it. Shallow waters with their greater variations of tempera- 
ture and consequent checks to shell secretion, tend to produce thin 
and lightly calcified shells, as do also the immense depths known to 
exist in some lakes, perhaps in this case due to the constant low 
temperature, much below the optimum for the species. 
The effect of a plethora of calcareous matter or of functional 
activity of the secretory organs is strongly exhibited in Anodonta 
cygnea v. incrassata, and is shown not only by the greatly thickened 
nacreous layer, but the plenitude of calcareous substance is often 
strongly indicated by a heavy deposit of the limy matter in the form 
of tufa, on the exposed posterior end of the shell. 
A characteristic specimen of this variety from a brook at Tisbury, 
Wiltshire, kindly given me by the late Mr. J. Pickering, w'eighs 
2,227 grains or 5T2 ounces, while an example of the typical form of 
equivalent size from Nagden, near Faversham, collected by the late 
Miss E. B. Fairbrass, weighed only 322 grains, or about one-seventb 
the weight of the variety incrassata. Rough and deep waters, 
although not profound depths, are both said to tend to produce thicker 
and stronger shells than those developed in shallow and more 
tranquil waters. 
Some species, as Unio margaritifer and Neritina fluviatiUs, 
appear able to extract the necessary lime carbonate to form thick 
and heavy shells even from the waters of gi’anitic districts, whilst 
