580 Records of the Indian Mtiseuni. [Vol. XXII, 



able number of specific names have been given to varieties and 

 phases of /. cxustus and we believe that all those forms, 

 assigned to Planorbis {s.s.) by Preston are reallj' sjaionyraous , 

 with the possible exception of P. Jiindu, Clessin. As to this form 

 nothing is known beyond the original description and figure, 

 and we doubt whether it is really Indian. There is, therefore, at 

 present no evidence for the occurrence of the true Planorbis in 

 India. 



Distribution of the Genus. — The type-species occurs not only 

 throughout the plains of the Indian Empire east of the Indus, 

 but also in Siam, the Malay Peninsula, French Indo-china and 

 Sumatra, whence we have recently examined numerous specimens. 



Indoplanorbis exustus (Desh.). 



1834. Planorbis exustus, Deshayes, Voy. Bell, hides Orient. Zool., p. 



417, pi. i, figs. 11-13. 

 1S36. Planorbis indiciis, Benson, Journ. As. Soc. Bengal V, p. 743. 

 1856. Planorbis coromandeliciis, Clessin, Mart. Chemn. Conch.-CaT)., 



Limnaeacea XVIII, p. 43, pi. vi, figs. 14-16, 20-22. 

 19:8. Planorbis exitstus, Annandale, Pec. Ind.'Miis. .KIV, p. iii, pi. 



xi, figs. I, la. 



In the paper cited above, one of us referred to I. exustus as 

 an example of a species which, comparatively speaking, was neither 

 variable nor plastic. When series from many different habitats 

 are examined it becomes evident that this statement should have 

 been qualified by some such phrase as "in normal circumstances." 

 Specimens from ordinary ponds and swamps in India, Siam and 

 Sumatra are very much alike, but those from pools subject to great 

 changes in physical conditions or containing water of very abnor- 

 mal chemical composition respond readily in peculiarities of shell- 

 structure. In the collection belonging to the Indian Museum Mons. 

 Iv. Germain has found a number of peculiar phases, some of which 

 he has described as varieties. 



An instance has recently come under observation in which a 

 distinct seasonal change has been noted in the shells found in a 

 certain pool situated on a small island and liable to considerable 

 vicissitudes. This pool, which lies in the middle of Barkuda, a 

 rocky island in the Chilka Lake, is a small artificial pond dug in 

 the rock. In the latter part of the rainy season and for some time 

 after it, roughly from the middle of July to December, it contains 

 from 12 to 15 feet of water which is only very slightly brackish 

 and remains fairly cool, but by April has sunk to a small puddle 

 of saline water heated by the sun to a high temperature. The 

 early showers which precede the monsoon in May and June fill up 

 the pond again, not of course to its level in the rainy season, but 

 sufficiently to give a depth of 4 or 5 feet of water and to reduce 

 the salinity very considerably. The pond has no visible aquatic 

 vegetation at any season, but I. exustus and a form of Liinnaea 

 luteola abound on the mud with which its rocky basin is deeply 

 covered. Specimens of the Planorbid taken in August are fairly 



