MR. H. F. BLANFORD ON THE DESCRIBED FORMS OF TANALIA. 



609 



Smnmary. 



To sum up tlie evidence above given: 



1st. When we class specimens according to any selected cliaracter (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 elimi- 

 nated by specimens of intermediate characters when we attempt to apply such classifica- 

 tion 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, some- 

 times affecting all. 



4th. Eorms with average characters are the most numerous, — those with extreme mo- 

 difications of character, such as greatly extruded spires, spiny ornaments, or of extremely 

 small or large dimensions, being comparatively rare. 



Prom the above considerations but one deduction can be drawn, viz. that the numerous 

 described forms of Tancdia (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 muzale, 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 Melcmia, from which indeed Tanalia, Faludonms, and 

 PUlopotamis differ so little, that it appears to me that all should rather be regarded as 

 sections of the genus Melcmia than as distinct genera. The structure of the oper- 

 culum, upon which alone their generic distinction depends, is variable both in F/iilopo- 

 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, we might ]'egard the group as 

 affording an instance of variable structure in an organ usually constant, the tendency to 



