1921.] N. Annandale & R. B. S. Sewell : Vivipara. 281 



numbers and at the end of July several were collected and 

 measured, and the results obtained are given in fig. 20. Tlie 

 ineasurement taken was the maximum height as defined above, 

 and the individuals fall into a well-defined regular group, the 

 measurenient ranging from 8 mm. to 14 mm. and having an 

 average of 11-3 ram. Since all these examples were less than 

 three months old, we get some idea of the very rapid growth that 

 takes place in early life. At the same time a number of adult 

 examples from the pond in the Museum compound were measured 

 and were found to fall into a group having a length measurement 

 of 20 mm. to 27 mm. This I believed to represent the size 

 attained after one year of life, and, in order to check this, 

 between July 26th and August 2nd, 1920, 409 examples from the 

 same pond of all sizes except the very smallest, which had 

 obviously only recently been born, were collected and measured. 

 The results are given in fig. 21. It will be seen that we have 

 two well-defined groups with their maxima corresponding to a 

 height measurement of 15 mm. and 24 mm. respectively. 



The members of the first group correspond very closely both 

 as regards size and degree of development with the examples 

 hatched and reared in the experimental tanks in 19 19. They are 

 somewhat larger, but the experiments of Semper (1874), De Varig- 

 ny (1894), and others have shown that growth is more rapid under 

 favourable conditions and in large areas of water with efficient 

 natural aeration than it is under artificial conditions in small 

 aquaria, and it seems reasonable to conclude that the individuals 

 comprising the group of the 1920 brood were approximateh^ three 

 months old and had been born about April. This rapid rate of 

 growth, from 3 mm. in height when born to 12-15 mm. at approxi- 

 mately three months, corresponds closety with the results obtained 

 by Lj'on {vide Baker, 191 1, p. 51) in which examples of Limnaca 

 (Galba) reflexa, measuring 2'00 mm. at 6 weeks old, attained to a 

 height of 5-10 mm. at 12 weeks and 26-o-28'5 mm. at one 3'ear 

 old, or by Woodruff (he. cit.) who found that examples of Limnaea 

 (Radix) auricularia increased from 075 mm. in height when born 

 to Ti'50 mm. at 4 weeks. As age progresses, the rate of growth 

 naturally becomes slower, since other and equally important pro- 

 cesses are going on in the young individual, especially the attain- 

 ment of sexual maturity. 



The second group, having an average height of 24 ram., 

 corresponds exactly with the adult examples taken from the pond 

 in August, 1919. 



It seems clear that these two groups of Vivipara bengalensis 

 correspond respectively to the 19 19 and 1920 broods, but there 

 are indications of a still further group having an average height 

 of 29-30 mm. ; this however does not appear very clearh' in the 

 chart owing to overlapping with the group of the 19 19 brood. These 

 large individuals, which were much fewer in number than those of 

 the preceding group, I take to represent individuals who have 

 survived for a further period of one year and who represent the 



