166 MR. H. F. BLANFORD ON THE RELATIONS OF 
conicus, Gray, Tanalia aculeata, Chemn., and Philopotamis 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 Ganga, 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. Tanjoriensis, 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 Paludomidee 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 
really 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 
decussata, Reeve, fig. 10, only differs from this last in having the spiral structure more 
distinctly developed. And, finally, from 7. 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, Reeve, 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 2). | 
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 
