608 
FORMS 
Between these extreme mo 
P. Bengalensis), at other times almost entirely concealed. 
difications every intermediate degree of exsertion may be met with ; and 
collected on the same spot (so far as my 
experience goes) 
specimens 
is always 
perceptible in this respect. The smaller varieties have, as a rule, the highest spires ; at 
least I have seen 
Paludiniform 
pies of the larg 
forms : and the nodulose 
varieties are less variable in the spire than those with simpler ornamentation, though even 
the most spiny forms exhibit considerable differences. 
In some specimens of a nearly smooth variety from a feeder of the Kelany Ganga be- 
tween Ambegammoa and Kitoolgalle, the upper whorls are exposed to considerably below 
the swell of the whorl ; but from the same spot I have specimens in which the suture coin- 
cides with the middle of the upper whorl, and others of intermediate degrees of inclination. 
Some specimens which I received from the Orobokka, in the Southern Province, exhibit 
a similar degree of variability. 
The following comparative measurements, in inches and lines, of specimens of spiny, 
ribbed, and large and small smooth forms exhibit the extremes of variation I have met with. 
Highest spire 
Spiny varieties \ Intermediate 
Lowest spire 
Highest spire 
Ribbed varieties 1 Intermediate 
Lowest spire 
Highest spire 
Large smooth variety -\ Intermediate 
Lowest spire 
f Highest spire 
Small smooth variety, from Kitoolgalle j. Intermediate 
( Lowest spire 
Diameter. 
Ratio of diam. 
to spire = 100. 
in. 
1 
t 
1 
lin 
1 o 
1 2 
1 3 
1 
1 2 
1 1 
9 
9 
11 
80 
90 
94 
85 
90 
100 
85 
96 
100 
75 
81 
91 
The above measurements can only be taken as roughly indicative of the exsertion of the 
dilatation of the shell, and 
spire 
as the ratio of height to diameter varies also with the 
the former is but little affected by a sHght exposure of the upper surface of the whorls. 
Series III. a and III. b of the Plate illustrate the variability of the spire in large smooth, 
and ribbed forms, respectively. 
4. Size of the Shell. 
In no respect do specimens of Tanalia vary more than in size. In this character, 
again, all the varieties (classed according to sculpture) exhibit a great range of difference, 
as is shown by the following four series of measurements. The specimens of the first are 
from the Mahavelli Ganga, illustrating the variability of full-grown shells from the same 
stream ; those of the other three series are respectively spiny, ribbed, and smooth or faintly 
ribbed forms, selected from various localities. (The measurements are in inches and lines.) 
