282 Records of the Indian Museum. [Vol. XXII, 



1918 brood. The great majority of these large examples show a 

 well-marked ' varix ' across the middle of the bodj'-whorl of the 

 shell, thus indicating that there has been a period of arrested 

 growth followed by a subsequent increase in size. The distance 

 from the apex of the shell to the umbilical end of the varix 

 measures approximately 24 mm. which corresponds closely to the 

 average height attained by examples that are one year old, and it 

 is evident that the period of arrested growth corresponds to the 

 second winter of their life-history. The maximum length of life 

 of any individual appears then to be two years, but the vast 

 majorit)' of individuals die after one year. Each year towards 

 the end of the rains there is a very heavy mortality among the 

 moUuscan fauna of the ponds and tanks, etc., in this country. This 

 was first noticed in a period of exceptional drought by Dr. Annan- 

 dale, who called attention to it in his preliminary report to the 

 Government of India on the mollusc survey of the Madras Presi- 

 dency, but he attributed it to the partial drying up of the pools 

 and the consequent foulness of the remaining water. The same 

 mortality, however, occurred, though perhaps on not quite so large 

 a scale, in ponds in Calcutta in August, 1919, where no such 

 causative agent could be suspected, and it appeared to be a 

 natural phenomenon affecting many different genera of molluscs, 

 including Vivipara.' In V. bengalensis, the vast majority- of 

 individuals born in the preceding year die during this period, 

 only a few surviving for a .second year. This heavy annual 

 mortality among the freshwater molluscs is a phenomenon of 

 considerable antiquity, for Annandale (1920 {a), p. 53) has adduced 

 evidence that it was in existence in the Intertrappean (late Creta- 

 ceous) beds of this country. 



The sexual differences in the antennae of Vivipara bengalensis 

 render it easy to carrj' out an investigation regarding the influence 

 of sex on the individual. I have been quite unable to detect any 

 difference in the shape of the shell, but measurement of a number 

 of individuals of both sexes, collected haphazard from the pond 

 in the Indian museum compound"' shows very clearly that there is 

 a quite appreciable difference in height. In fig. 22, I have given 

 the measurements of 147 female and 57 male examples and it 

 shows that the average height of females of one year old is 25-0 

 mm. and of two years old 30*0 mm., whereas males of the corres- 

 ponding length of life have an average height of only 22'o and 

 27'0 mm. respectivel}'. 



Difference in size in the two sexes of Vivipara vivipara was 

 noticed as long ago as 1695 by Lister, and more recently Wood- 

 Mason (1881, p. 86) has recorded the same sexual character in 



J Vide Annandale and Sewell " Progress report on a Survey of the Frcsh- 

 ttater Gastropod Molluscs of the Indian Empire and of their Tremetode Para- 

 sites." Ind. Jourii. Med. Research \'III, p. 119. 



^ Examples living in this pond are considerably sm;iller than those found in 

 certain other localities. 



