igai.J N. Annandale & R. B. S. SeweLL : Vivipara. 



283 



examples of Vivipara crassa (Hutton) that he obtained and exam- 

 ined in Sj'lhet. This difference in size is also known to occur 

 in other genera and according to Cooke (1895, p. 134) "is markedly 

 the case in Lit'.orina, Bnccinitm, and a\l the Cephalopoda." It is 

 general^ assumed that the dift'erence in the two sexes is related 

 to the viviparous habit and is dependent on the necessity for 



1918 brood 



1919 brood 





mm. 



35-0 

 340 

 330 

 32-0 

 310 

 300 

 29^0 

 28-0 

 27-0 

 26^ 

 250 

 24-0 

 23-0 

 22-0 

 210 

 20K) 

 19 

 180 

 17-0 

 18-0 

 ISO 

 14-0 

 13-0 



J 



1918 brood 



1919 brood 



•••e«*««««« 



Fig. 22. — Measurements of 147 ? examples and 57 d' examples of Vivifnra 

 bengaleusis. 



increased space in the shell of the female in order to accommodate 

 the large numbers of contained young in utero.- If this were the 

 sole causative agent, one would expect to find the condition cons- 

 tantly present, but this expectation is not fulfilled in the Vivi- 

 paridae of this country, for in certain species no difference can 

 be detected in the two sexes, while in others there is a dift'erence 



