IQ2I.] N. Annandale & R. B. S. SewELL : Vivipara. 285 



belonging to the 1918 brood and examined by me in August, 

 1920 were eggs, in which development was in an early stage, 

 found in jttcro ; in five others the uterus contained a few embryos 

 in a comparatively advanced stage, with 3 to 3^ whorls in the 

 spire; and in the remaining seven the uterus was empty. The 

 average production of these examples was 28. On the other 

 hand nine females belonging to the 1919 brood 3'ielded an average 

 of I2'0 eggs or developing young, so that in spite of their 

 greater size, examples of the age of two years show very distinct 

 evidence of senile decay, and it is not improbable that in many 

 cases the young offspring found in utcro had been retained since 

 the previous breeding season. 



Under certain conditions of cold, drought, etc., Vivipara benga- 

 lensis appears to be capable of undergoing ' hibernation.' In Decem- 

 ber, 1919 I examined a series of examples that had been obtained 

 during that month by Mr. Sunder Lai Hora from the mud, in which 

 they had buried themselves, at the bottom of a pond in I,ahore. In 

 every case the uterus contained a certain number of live young, 

 which must obviously have been the product of the previous breed- 

 ing season, and which would doubtless have been set free during 

 the following season, in the event of the parent having been able 

 to survive. Annandale has pointed out that V . hengalensis is less 

 modified, especially in the structure of the operculum, for resisting 

 drought than the species of the V. dissimihs group (sub-genus 

 Idiopoma, Pilsbry). In a bottle full of specimens of V. hengal- 

 ensis and V. dissimilis, recently brought to Calcutta from the 

 Ganjani district of Madras, the difference in the behaviour of the 

 two species as the water became foul was very marked. The in- 

 dividuals of V. hengalensis crowded round the edge at the top of 

 the water with the aperture of the branchial chamber above the 

 surface and widely open, as though inhaling air, while those of 

 V. dis-^imilis closed their opercula tightly and sank to the bottom. 



A further interesting point brought out by a study of the 

 two sexes is the greater mortality among males during the period 

 following the attainment of sexual maturity. Out of the fifty 

 examples of the 1920 brood that were examined the proportion of 

 the sexes was 24 cf and 26 9 , so that at this period of life the num- 

 bers are approximately equal. A reference to fig. 21 shows that at 

 the end of the first year of life the proportion of the sexes was 

 203 9 to 510" or roughly 4:1. While at the end of the second 

 year of the life the proportion had become still greater and there 

 were as many as 44 9 to 6 cf or 8 : i. Wood-Mason (1881, p. 87), 

 when examining a series of examples of Vivipara crassa, found 

 that in seventj'-six specimens forty-six were females and onlj^ 

 thirty were males. He was, however, doubtful whether this 

 difference in the numbers was due to an actual minority in the 

 males or was merely the result of his collector having naturally 

 tried to secure the largest possible specimens, but in view of the 

 figures obtained by me in V. hengalensis I am inclined to believe 

 that we get a similar disproportion of the sexes in both these 



