TANALIA, PHILOPOTAMIS, AND PALUDOMUS. 167 



Philopotamis a similar modification obtains to a less extent, accompanied in Ph. glo- 

 bulosa with an incipient concentric growth. In Paludomus, on the other hand, the 

 growth of the operculum is, up to a certain point, that of a typical Melania : after the 

 formation of one and a half or two whorls it suddenly takes a different direction, and is 

 superseded by a strictly concentric growth, in which the chief addition is in the direction 

 of the outer or dextral margin instead of the inner as in Melania, Philopotamis, and 

 Tanalia. It would be arguing in a circle, and on an unproved generalization, to adduce 

 the comparatively wide range of Paludomus, as contrasted with that of Philopotamis and 

 Tanalia, in support of their generic diversity, on the supposition that successive grada- 

 tions of structural character argue successive appearance in time, and that the earlier 

 forms have the wider range ; but if the structural argument appear valid, the facts of 

 range will have much significance to those who admit the possibility of generic derivation 

 from preexisting types. 



The solid structure and depressed form of the shells of Paludomus, Tanalia, and Phi- 

 lopotamis may be held to indicate a closer relationship between these genera than exists 

 between any one of them and Melania. In these respects, however, the true Melanias 

 exhibit much variety. Paludomus brevis, D'Orb., is a Melania as regards its operculum, 

 although the shell has, on account of its form, been erroneously referred to Paludomus ; 

 and the Melania Hugelii* of S. India is almost as thick and depressed as certain Paludomi. 

 The genus Gyrotoma also, with the operculum of Melania, has the solid, depressed form 

 of Paludomus, yet would scarcely be classed therewith. 



With respect to habit, no marked line can be drawn between the different genera in 

 question. Tanalia aculeata is indeed never found in still water, nor, I believe, elsewhere 

 than in mountain streams ; but it is met with in the smallest perennial streamlets as well 

 as in the largest torrents : it is generally found adhering to rocks, but also frequently 

 crawling over sandy bottoms. Philopotamis globulosa has a similar range of habit, but 

 is apparently more restricted in geographical range. I have myself only met with it in 

 the Mahavelli Ganga and tributaries, and in the stream of the Balcadua Pass, nor have 

 I seen any specimens from the southern or eastern watershed of the hills. Ph. sulcata 

 occurs chiefly in the smaller streams on and among the hills, extending from an elevation 

 but little above the sea to a height of 3000 feet. Mr. Layard records that this species 

 sometimes climbs far out of the water ; but, though I do not question his accuracy, I do 

 not remember ever to have met with it myself in such a situation. The species of 

 Paludomus are, as their name imports, the most marsh-loving in their habits; but 

 P. chilmoides and P. Tanjoriensis, the only species I have myself collected, though 

 occurring in marshes and tanks, are at least equally or even more common in streams 

 and rivers, of both slow and rapid flow, on the hills and in the low country. 



* This species is from the upper waters of the Cauvery, where it has been rediscovered by Dr. Jerdon. The quoted 

 habitat, New Holland, is probably erroneous. 



