MR. H. F. BLANFORD ON THE DESCRIBED FORMS OF TANALIA. 605 



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 structm'e common to other 

 species of Gasteropoda in similar cases. [See Dr. Gray, Proc. Zool. Soc. 1854, p. 100.] 

 Two specimens of opercula in my collection seem to prove this. In the first a small portion 

 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 con- 

 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 character. 



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. violacea. 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 ; but 

 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. aculeata ever approach 

 the position it occupies in T. violacea. Further materials may, however, show that these 

 distinctive characters graduate off through intermediate forms. 



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 described 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 specific 

 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 types were such 

 as were shown to be very variable by other series, so that I could not doubt that fiu'ther 

 materials Avould eventually be found to supply the missing links. 



When, subsequently, I collected living specimens in the Mahavelli Ganga, tlie 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. 



