166 MR. H. F. BLANFORD ON THE RELATIONS OF 



conicus, Gray, Tanalia aculeata, Chemn., and PUlopotamis sulcata, Reeve, sp., are quoted 

 as the respective types of the genera. 



Two other genera have been formed from the species herein included, viz. Rivulina, 

 Lea, and Gang a, Layard; but there appears to be no distinction between the former 

 genus and Paludomus, the two species quoted under it being, so far as I can judge from 

 Mr. Cuming's authentic specimens, mere varieties of Paludomus chilinoides, Reeve, and 

 P. Taujoriensis, Chemn., while the latter is, as I have shown in my former communica- 

 tion, founded upon certain monstrous forms of Tanalia aculeata. 



In the June Number of the ' Ann. & Mag. Nat. Hist.' for 1856, Mr. Benson pointed out 

 that although the adult operculum of Paludomus has, as stated, a concentric structure, 

 much resembling that of Paludina, with which the genus had consequently been classed 

 by Dr. Gray, and subsequently by Mr. A. Adams, the embryonic operculum or the nucleus 

 of the adult is spiral, resembling that of a Melania. He also remarked that the subspiral 

 structure of the operculum of Philopotamis indicated an affinity with Melania, but ex- 

 pressed a doubt whether Tanalia, with its unguiculate operculum, should be referred to 

 the Paludomidae at all. 



Now remembering that no essential difference of structure has been noticed in the 

 animals of these genera, and that, with the exception of certain differences of habit, upon 

 which I shall remark presently, the only assigned generic characters held to distinguish 

 them from each other and from Melania are those of the operculum, let us see what these 

 i -ally amount to. PL XXVII. fig. 8 represents, on an enlarged scale, the operculum 

 of a Ceylonese specimen of Melania spinulosa, Lamk., a common Eastern species, and, 

 on the point in question, a fair representative of the genus. The structure is paucispiral, 

 and the nucleus subbasal. Fig. 9 is the operculum of M. lirata, Bens., also enlarged : in 

 this the structure is less distinctly spiral, and the nucleus is almost marginal and basal. 

 In fig. 13, the operculum of Tanalia violacea, Layard, a subspiral structure is still appa- 

 rent, but the nucleus is marginal and dextrally subbasal. The operculum of Philopotamis 

 decussatct, Reeve, fig. 10, only differs from this last in having the spiral structure more 

 distinctly developed. And, finally, from T. violacea we pass to T. aculeata, fig. 14, in 

 which a trace of a subspiral structure is only perceptible towards the nucleus ; while 

 from Ph. decussata we proceed through Ph. sulcata, fig. 11, to Ph. globulosa, figs. 12 a & b, 

 which is the most aberrant form presented in the genus. We have thus tolerably perfect 

 series from the typical Melania operculum to the extreme forms of Philopotamis and 

 Tanalia. The operculum of Ph. nigricans, Eeeve, is more nearly related to that of 

 Melania than any of the above, and indeed some forms are scarcely distinguishable 

 generically : fig. 15 a affords an instance of this ; but in others again the Philopotamis 

 structure is more developed (fig. 15 b). 



It might seem at first sight that the operculum of Paludomus is, in an abstract mor- 

 phological point of view, merely that of a Philopotamis with the nucleus pushed over 

 towards the other side; but it is, I think, essentially different, or rather, the digression 

 from the Melania type of structure has taken place in a different direction. The passage 

 from Melania spinulosa to Tanalia aculeata may be regarded as a gradual unrolling of 

 the opercular spiral, which becomes nearly obsolete by successive gradations, while in 



