Summary To sum up the evidence above given: 1st. When we class specimens according to any selected character (such as sculpture), we find them variable in all the others, especially in form and size; and if we attempt to multiply species still further, e. g. taking two selected characters as the basis of our classification, we still find them variable in the remaining characters. 2nd. Whatever character or characters we select as the grounds of classification, the differences observable when but a small number of specimens are compared, are eliminated by specimens of intermediate characters when we attempt to apply such classification to a large number from various localities. 3rd. Specimens collected on the same spot are always variable to some extent, and sometimes greatly so, the variation being sometimes confined to one character, sometimes affecting all. 4th. Forms with average characters are the most numerous,—those with extreme modifications 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 Tanalia (excepting, possibly, T. violacea) are varieties of one species. I have in these remarks confined my observations 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, Paludomus, 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 genera. The structure of the operculum, upon which alone their generic distinction depends, is variable both in Philopotamis and Tanalia, and is, as we have seen, also variable in the species and varieties of Tanalia. Indeed, accepting the views of Mr. Darwin, we might regard the group as affording an instance of variable structure in an organ usually constant, the tendency to vary having survived generic and specific differentiation and obtaining to some extent in coexistent forms of the same species. Reverting to the subject upon which I remarked at the commencement of this paper, viz. the endemic character of the land and freshwater molluscan fauna of Southern Ceylon, it appears that in the case of Tanalia, as also without doubt in that of Aulopoma (two of the peculiar genera), the number of species is really far less than has been represented, that of the former genus being (possibly two) probably but one, that of the latter one only. Of Philopotamis I have met with three species; and two others have been described by authors, but are probably only varieties. Of Cataulus there appear to be five or six and possibly more species; but this genus is not strictly endemic, having a single described representative in the Nicobar Islands. Both Philopotamis and Cataulus still require more strict investigation than the limited materials at my disposal have enabled me to give them.
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Sep 1, 1995
Christopher M Moore 
Open Access