86 Mr. J. Wood-Mason on the 
marked difference in size presented by the shells of a species 
of Paludina which lay in thousands at the bottom of the 
broad and shallow ditches close to the river-bank: shells of 
two distinct sizes, a larger and a smaller (not, so far as I 
could see, graduating into one another), were observed. ‘This 
difference of size evidently not depending upon age, but 
being probably sexual, I determined to investigate the matter 
as soon as opportunity offered. 
On my arrival at Silchar my native collector brought me, 
amonest other freshwater mollusks, five specimens of a Palu- 
dina*, belonging to a totally distinct species from the one 
observed at Balaganj and Panchuganj. Of these, three of 
the same size were smaller and rather less ventricose than 
the other two, which also agreed with one another in size. In 
order to ascertain whether any difference in the external form 
of the animals accompanied the obvious difference between 
their shells in point of size, I placed the specimens in a plate 
of water. In a few minutes the mollusks emerged from their 
shells; and I at once noticed that, while the acuminate ten- 
tacles of the two larger ones were both quite straight, the right 
tentacle in each of the three smaller ones was strongly bent 
outwards and inwards upon itself, or hooked and somewhat 
swollen, and rather more darkly pigmented than its fellow of 
the opposite side or than the tentacles of the larger ones—an 
observation which suggested the suspicion that in the former 
case one had to do with males, and in the latter with females. 
To place the matter beyond all doubt, I dissected two of the 
smaller and one of the larger specimens; and I found in the 
genital gland and duct of the former two forms of sperma- 
tozoa in all stages of development, and in the uterus of the 
latter ova containing fully-formed embryo Gasteropods with 
foot, operculum, spirally-lined soft nautiloid shell, upturned 
proboscis, and all complete. 
Considering it desirable that this observation should be 
verified by the examination of a larger series of examples, I 
sent the collector to the river for a fresh supply; and he soon 
returned with seventy-six specimens. ‘These were placed in 
a basin of water and sorted, like those previously obtained, 
into two sets, according to the form of the right tentacle ; 
each set was then carefully gone over to see that the sorting 
had been correctly carried out; and finally the sex of several 
individuals out of each set was determined by dissection and 
microscopic examination, with the result of completely con- 
* T am indebted to my friend and colleague Mr. Geoffrey Nevill, 
C.M.Z.S., for the information that the name of this species is P. crassa, 
Hutton. 
