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 Paluwdomus, on the other hand, the 
growth of the operculum is, up to a certain point, that of a typical Me/ania: 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superseded by a strictly concentric growth, in which the chief addition is in the direction 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 Baleadua Pass, nor have 
I seen any specimens from the southern or eastern watershed of the hills. PA. 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. chilinoides and P. Tanjoriensis, the only species I have myself collected, though 
ing 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. 
Z2 
