137 
The two methods, intercalation and rib-splitting, may 
occur alone in respective individuals, or both in the same 
example, and there may be any conceivable ratio between 
the two methods in different specimens. 
The contour of the ribs may vary greatly. They may be 
at their inception narrow and comparatively high, and may 
so continue throughout their length. Or after some increase 
in size they may begin to decline in height until they almost 
fade out and leave the anterior part of the shell nearly 
smooth. Their shape may completely change; whereas, at 
first, they may have concave interstices so as to resemble a 
fluted column, the ribs may widen out and become convex, 
while their interstices become reduced to narrow, shallow 
grooves between broad, approximate, rounded ribs. 
The number of ribs at the ~posterior end is very variable. 
If multiplication of coste occurs, it is plain that the older the 
shell and the more truncated, the greater will be the number 
of ribs at the posterior extremity ; and if such multiplication 
always began at the same stage of growth and was equally 
rapid, the number would always be greater with a greater 
truncation. But such is not the case, hence the number of 
ribs at the hinder end varies widely. Six is the fewest I have 
found. But there may be any number beyond this up to 
fourteen, which is the most yet observed. These larger num- 
bers are by no means restricted to examples with much trun- 
cation, nor is there any definite proportion between the num- 
ber of ribs and the diameter of the shell: some of large dia- 
meter at the truncation have but few, and vice versa. A 
diagnosis framed upon the number of ribs would be baseless. 
Pilsbry suggests the typical form is hexagonal ; probably he 
is right, but usually there are more than six coste. 
The anal appendical tube is wanting in most cases, even 
during life. When present it may be two or three milli- 
metres long. It may exist when the shell is young and nar- 
row, and be wanting when old and wide; possibly it may have 
been broken off. It is central and most frequently in the 
axis of the shell. But it may be distinctly out of the axis, 
joined at an angle so as to point markedly towards the con- 
cave side, or slightly towards the convex, and in one it is 
funnel-shaped instead of cylindrical. These circumstances 
confirm the suggestion of its being an outgrowth subsequent 
to truncation, and not merely a residual inner layer of the 
shell after the outer portion has been absorbed. 
The radula (pl. xxvi., figs. 14a, b 
tively large, and contains fifteen ro 
formula 1.1.1.1.1. The r 
as high, is thickened al 
» ¢. d)-is compara- 
fi ws of teeth, with the 
achidian tooth is about twice as wide 
ong its free edge, and thinned along 
