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[ 165 ] 



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X. 0# the Relations of Tanalia, Philopotamis, and Paludomus ; with a review of the 

 Cingalese Species of the latter Genera, By H. P. BLANFOKD, F. G.S. Communicated 

 by Dr. J. D. HOOKER, F.H.S., F.L.S., fyc. 



(Plate XXVII.) 



Read June 18th, 1863. 



a paper read at the meeting of this Society in June last, I expressed an opinion that 

 the Cingalese genera Tanalia and Philopotamis* are more closely related to Melania than 

 has been held by several recent writers, and that in fact they should be regarded rather 

 as sections of that genus than as distinct genera. The object of the first part of the 

 present paper is to substantiate that view by showing that the opercula of Philopotamis 

 and Tanalia, upon the structure of which generic distinction has been based, so far from 

 being distinct in type, really present modifications of the subspiral operculum of Melania, 

 from which the digression is serial and gradated in the different forms (species and 

 varieties) included under those genera. While, however, insisting on the facts of affinity, 

 I should at the outset so far modify the above general assertion as to admit that the 

 question of genus and subgenus is to a great extent one of opinion, depending upon what 

 amount of difference be held to constitute a family, genus, or subgenus. By those who 

 regard such a form as Melania variabilis, Bens., with its largely spiral operculum, as a 

 true Melania, Philopotamis and Tanalia should be treated as subgenera, their aberration 

 in one direction from the type not being greater than that of M. variabilis in another 

 direction. To those, on the contrary, who, with Messrs. Gray and Adams, regard most of 

 the Lamarckian genera as families, Philopotamis, &c., will rank with the typical Melanias 

 as nearly allied genera of one family. 



In the second part of the paper I shall review the Cingalese species of Paludomus and 

 Philopotamis ; the latter genus is, so far as we know at present, restricted to Ceylon. 



PAUT I. 



It is unnecessary to review in detail the various opinions that have been expressed with 

 regard to the affinities of Tanalia, Philopotamis, and Paludomus. The last-mentioned 

 genus was distinguished from Melania by Swainson on account of the concentric structure 

 of the operculum, and Tanalia and Philopotamis were subsequently separated therefrom 

 by Gray and Layard, the former genus having a trigonal operculum with a marginal 

 nucleus, the latter a subspiral operculum with a submarginal nucleus. Paludomus 



* Paludomus was also included in this remark, but reconsideration leads me to regard it as more distinct than either 

 of the above. See posted. :rr * \*i-\ /* f * '7 



VOL. XXIV. Z 



