1918.] N. Annandale : MoUvscs of the InU Lake. 159 



form that now lives in ponds and swamps both in the Inle and the He-Ho 

 basins, but have the spire constantly a little more elongate and slender. 

 Unfortunately we have no information as to the type of habitat in which 

 they were found. Both in them and in the Inle pond-form the most 

 important of the specific characters is well marked. The character iS' — 

 not only is the whole margin of the aperture thickened but on the 

 outer part of the shell there is also a thick ridge or varix running 

 almost parallel to the thickened margin across the body-whorl. In the 

 type-specimens and in those of the Inle pond-form the ridge lies very 

 near, but distinctly separated from the rim of the aperture. In the 

 Inle Lake another race is predominant. The shell attains a larger size, 

 is usually of a brighter and yellower colour, is invariably narrower and 

 more elongate, and has the varix separated from the lip by a consider- 

 ably greater though very variable space. When the shell is fully devel- 

 oped the ridge no longer lies parallel to the rim of the aperture bvit has 

 a much less marked downward convexity. To this form I have given 

 the name iacustris, regarding it as a race or subspecies of H. nassa. I 

 cannot find any anatomical difference between it and the latter. A 

 third race is found in fair abundance in small streams at Thamakan 

 some distance west of He-Ho. This locahty lies about 400 feet higher 

 than He-Ho, and 1200 feet higher than the Inle Lake. For the Thama- 

 kan race I have proposed the subspecific name rividicola. The shell 

 agrees with the typical form in its general structure, but is distinctly 

 thinner and more conoidal ; in size it slightly exceeds it. Yet a fourth 

 race (subspecies distoma) occurs, or rather occurred, in the Inle district. 

 It is found both fossil and subfossil in the different deposits of the He-Ho 

 plain. The shell is small, thick, and moderately elongate, but its chief 

 character lies in the very close proximity of the varix to the rim of the 

 aperture. So close, indeed, do they lie that the two thickenings form 

 together little more than a single ridge divided transversely by a narrow 

 groove. 



In this species, therefore, and in its ally H. avarix we have a most 

 interesting series illustrating the gradual accentuating of a generic 

 character. In H. avarix the actual rim of the mouth of the shell is 

 thickened, but there is no varix. In the fossil and subfossil race of H. 

 7iassa the varix is beginning to be differentiated from the thickened rim 

 of the aperture, in the Thamakan race the process has been carried 

 further, in the typical form of the species further still, while in the 

 lacustrine form the two ridges have little connection. 



In the Inle Lake the local phase of the typical form of the species 

 and the lacustrine race occur together in the marginal zone, in which 

 conditions are not very different from those to be found in a large pond 

 full of vegetation such as the former usually effects, but in the open 

 central region only the lacustrine race occurs. This race grows to a 

 larger size in the intermediate zone of the lake than it does in the centre 

 of the central region or in the marginal zone, but I have not been able 

 to find any other difference in shells from different parts of the lake, 

 except that examples from clear water are usually more transparent 

 than those from places where it is at all turbid or discoloured. It 

 is difficult, moreover, to be sure that the latter peculiarity is not 



