1918.] N. Annandale : Molluscs of the Inle Lake. 107 



Preston describes the shell as follows : — 



" Shell rimate, acuminately ovate, rather solid, polished, shining, pale yellowish 

 horn colour ; whorls 4, shouldered above, marked with rather coarse lines of growth: 

 sutures deeply impressed ; columella obliquely and diffused above into a thick callus 

 which joins the upper margin of the peristome ; peristome simple ; aperture ovately, 

 inversely auriform." 



He also refers, on the authority of the Rev. E. W. Bowell, to the 

 peculiar form of the upper jaw, the central part of which is produced 

 into a blunt beak-like projection, and to the Planorl)is-\\\iQ appearance 

 of the radula. Mr. Bowell even suggested the erection of a new genus 

 on account of the former character. 



I mention L. howelli here because we found in the Inle and He-Ho 

 basins a series of fossil and recent shells that difTered considerably in 

 different deposits and zones but formed a very regular gradation be- 

 tween a shell that resembled that of L. howelli in certain characters 

 but differed from it considerably in others, and one that had little out- 

 ward resemblance to it. The only living phase in this series, moreover, 

 though far removed from the Tibetan form in shell-characters, proved 

 on comparison to be identical with it in buccal armature. L. howelli 

 is probably itself a modified form, living as it does at an almost unique 

 altitude, but it is, to say the least, highly probable that still further 

 links might be found between it and the various phases I have here 

 grouped together under the name L. shanensis. 



Limnaea shanensis, sp. nov. 

 Plate X, figs. 5, 8 ; plate xi, figs. 2, 3. 



The shell in all the phases of this species is much narrower than in 

 L. hoivelli, the aperture is less patent, the peristome projects less, the 

 spire consists of two instead of three whorls, the whorls are not at 

 all flattened above and the suture is more sinuous and more oblique. 

 These characters become intensified with the gradual evolution of 

 what appears to be a true lacustrine deep-water type, but certain 

 features are always retained, viz., the complete aperture, the imper- 

 forate but rimate condition of the shell, the short, rather blunt spire, 

 impressed suture, ovate form, coarsely developed growth-lines and 

 small size. More important than any of these is the structure of the 

 jaw and radula in the only living phase. Each phase, of which I can 

 distinguish four, is or was associated with a definite type of environ- 

 ment. I will refer to them alphabetically and describe them seriatim. 

 The complete gradation is better shown on plate x than can be 

 described in words. 



Type-sfecimen (phase A). No. M. 11059/2, Zoological Survey of 

 India {Ind. Mus.). 



Phase A. 



In this phase the shell is nearer that of L. howelli than any other, 

 but there are distinct and constant features in the much narrower form, 

 more oblique suture, longer and narrower aperture, etc. The shell is 

 fairly thick and its growth-lines are very coarse. It is of approximately 



