150 Records of the Indian Mitseu'm. [Vol. XIV, 



the chief differences between the shells are that that of the still-water 

 form is considerably broader and has a more patent aperture and a 

 shorter spire than that of the running-water form. 



The other two Inle species of Limnaea {L. shanensis and L. mimetica) 

 are proved to be related to L. howelli rather by the structure of their jaws 

 and radulae than by. any very close resemblance in the shell. The 

 peculiarities of the buccal armature of L. howelli are so great that it has 

 been proposed to erect a new genus for its reception. The similarity in 

 this respect between L. howelli and the only living phase of L. shanensis 

 is so close as to amount to a practical identity. We know of four phases 

 of L. shanensis, three of which are fossil or subfossil, while one survives 

 in the lake. Information is available as to the habitat in which each 

 phase lives or lived. The shell in the four phases represents a gradual 

 and almost even series from a form of normal shape, not very far re- 

 moved from that of L. howelli, but narrower and with a smaller spire. 

 This phase lived in an open but rather shallow lake at an altitude of 

 about 3,800 feet. The next phase, which is distinctly narrower and 

 has the spire rather more reduced, lived in the same neighbourhood and 

 at the same altitude, but in conditions that were rather paludine than 

 lacustrine. The third phase, which still lives in the Inle Lake at an 

 altitude 800 feet lower, inhabits the marginal zone amidst decaying 

 vegetation. While going further in the direction of narrowness and 

 reduction of the spire, the shell differs from all the other phases in its 

 thinness. The fourth phase lived in the open part of the Inle Lake, 

 possibly at a time when the water was very much deeper than it is 

 now. The shell is in all respects, except that it is rather thick,^ that 

 of a typical deep-water form of the genus, and is by no means remote, so 

 far as shape is concerned, from L. mimetica, the only species that now 

 lives in the central region of the Inle Lake. 



In every particular, including thinness and paleness of shell and lack 

 of pigment in the soft parts, L. mimetica is a typical deep-water form, 

 although it survives in water that is nowhere more than 12 feet deep. 

 Its actual descent from the extinct deep-water phase of L. shanensis 

 is negatived by the divergence in the structure of the radula and 

 columella ; the differences are greater than those between that phase 

 and L. howelli. L. mimetica and the extinct Inle form have, however, at 

 any rate followed a similar course in the line of descent. The shells in 

 both cases are greatly reduced in size ; the spire is relatively small ; the 

 whole shell is narrow, and the aperture, though by no means expanded, 

 is relatively of great size. The differences between the shell of these two 

 forms on the one hand and that of the Tibetan L. hotvelli on the other 

 are strictly comparable to those between L. abyssicola and L. foreli 

 of the deeper parts of the Swiss Lakes and the common European 



1 It has struck mc as not improbable that dead shells in strongly calcareous water 

 may grow thicker by the equal deposition on their surface of salts of lime. Geologists 

 whom I have consulted on the subject are not agreed as to tlie possibility of this but 

 one distinguished member of the Geological Survey of India tells me that he believes 

 that it frequently occurs. 



The ]i()sitinn of the shells of phase D on or near the surface of the mud is dilficult 

 to account for if they lived at a period previous to the filling in of the lake. There is 

 no evidence that the watcr-kvel has sunk to any great extent. 



