MR 
CO! I 
xt 1 f Height 
* \ Diameter 
No. 2. 
No. 3. 
f Height 
\ Diamete 
Height 
Diameter 
No. 4 
Height 
Diameter 
o. 
in. lin. 
1 14 
1 3 
1 1 
1 0£ 
10£ 
o n 
6 
7. 
8. 
in. lin. in. lin.! in. 1 
1 3 
1 2 -i 
] 6| 
1 2 1 3i 
1 
] 4 
11 1 
11" 
H 
1 3 
1 2j I 4 
1 5 1 
1 2 
1 11 
) 9 
1 7 
1 5 
1 5 
9. 
in. lin. 
1 8 
Summary. 
■ 
To sum up the evidence above given: 
1st. When we class specimens according to any selected character (such us sculptui 
we find them variable in all the others, especially in form and size ; and if we attempt 
multiply species 
still further, e. g . taking two 
selected characters as the basis of our 
the 
classification, we still find them variable in the remaining characters. 
2nd. Whatever character or characters we select as the grounds of classification, 
differences observable when but a small number of specimens are compared, are elimi 
nated by specimens of intermediate characters when we attempt to apply such classifies 
tion to a lar^e number from various localities. 
3rd. Specimens collected on the same spot are always variable to some extent, 
(1 
sometimes greatly 
times affecting all. 
the variation beinsr sometimes confined to one character, some 
o 
4th. Forms with average characters are the most num 
those with extreme mo 
difications of character, such as greatly extruded spires, spiny ornaments, or of extremely 
small or large dimensions, being comparatively rare. 
From the above considerations but one deduction can be drawn, viz. that the numerous 
described forms of Tmutlia (excepting, possibly, T. violacea) are varieties of one species. 
I have in these remarks 
fined my obser 
to the shell; but it is upon the 
characters of the shell, and these alone, that specific distinctions have been founded. So 
far as I have had opportunities of observing the animal in the living state, it varies but 
little. The colour is slaty blue on the back and muzzle, brown towards the edges, and 
a pale cinereous grey or flesh-tint on the creeping-disk, the variation being dependent on 
the relative individual abundance of orange-pigment granules. The dorsal fold of the 
mantle is fringed as in the genus Melania, from which indeed Tanalia, Paludowus, and 
Philopotamis differ so little, that it appears to me that all should rather be regarded as 
sections of the genus Melania than as distinct g 
The structure of the oper 
culum, upon which alone their generic distinction depends, is variable both in Philopo- 
tamis and Tanalia, and is, as we have seen, also variable in the species and varieties of 
Tanalia 
Indeed, accepting the views of Mr. Darwin 
misht 
e> 
o 
d the group as 
affording an instance of variable structure in an organ usually constant, the tendenc 
