MR 
005 
is central or subcentral, and the structure consequently concentric; but this modification 
is only met with in those cases in which there is good reason to believe that the original 
operculum has been lost, and a new one formed on a plan of structure common to other 
species of Gasteropoda in similar cases. [See Dr. Gray, Proc. Zool. Soc. 1851, p. 100.] 
Two specimens of opercula in my collection seem to prove this. In the first a small port ion 
only of the original operculum has been lost, and the older part has the usual structure 
with a dextral nucleus, the newer part being concentric, as in the monstrous specimens 
above alluded to. The other case is still more conclusive. The original operculum, which 
has been only partly detached from the animal, and not destroyed, is of the usual structure ; 
the newer, which has been formed beneath and finally fused on to the former, has the eon- 
centric structure extending up to where the older operculum still adhered to the animal. 
The concentric opercula, moreover, are always more or less uneven and irregular, which 
is never the case with the normal forms. There is therefore no reason to regard this 
structure as a specific, far less as a generic or subgeneric charact r. 
Among the numerous forms of Tanalia that I have examined, but one departs so 
widely from the others as to admit of being regarded, even provisionally, as a distinct 
species, viz. that described by Mr. Layard as T. riolacea. The characters which distin- 
guish this from all others are the peculiar form of the operculum above noticed, and the 
dense violet colouring of the interior of the shell, as well as the small size of the latter. 
These two last characters are, indeed, variable in the other species of the genus; and 
did they alone distinguish T. violacea, I should feel great hesitation in separating it ; hut 
I have not seen in any other form an operculum similar to that of this little species, nor 
does the position of the nucleus in the numerous varieties of T. amleata ever approach 
the position it occupies in T. violacea. Further materials may, however, show that these 
distinctive characters graduate off through intermediate forms. 
*«.i^«.^ v," v^~~ 3 
T. aculeata, Gmel. sp. 
The remark of Major Skinner, that it is impossible to separate the numerous forms of 
Tanalia into species or varieties with definite limits *, I found to be fully confirmed by my 
own experience when I examined his collection, which comprised about 3000 specimens 
from various localities. Typical specimens corresponding to most of the (limbed species 
were easily selected ; and it would have been equally easy to double the number of distinct 
forms by selected specimens differing as much inter se as most of those to which speciti. 
names have been given ; but when any attempt was made to classify the collection, such 
typical forms were found to graduate into each other; and in those cases in which the 
collection did not furnish forms strictly intermediate, the characters of the type* were such 
as were shown to be very variable by other series, so that I could not doubt that further 
materials would eventually be found to supply the missing links. 
When, subsequently, I collected living specimens in the Mahavelli Ganga, the Kelany 
Ganga, and some other streams, and when I examined other collections classed according 
* T. violacea must be excepted in this and the following remarks. 
