8 INTRODUCTION. 
me is so close that it appears to cut the beads into two parts, and 
another variety of a shell similar in almost every other respect, has 
the first groove-line in the centre of the body-whorl. When the 
ribs are interrupted at a distance from the suture, these short ribs 
are termed riblets, and they vary from being a little longer than 
broad, until they reach beyond the centre of the whorl. In a 
group of four specimens just examined, all the varieties with the 
exception of one are plainly ribbed, without being coronated, but 
the Jast has distinct beads ; thus we have a change from one to the 
other in specimens which are apparently merely varieties. From 
these observations taken from the shells, and not intended to sup- 
port or illustrate any theory, it is very easy to see that instead of a 
fixedness in the characters used for the determination of species, 
exactly the opposite appears to be the case; the specimens presenting 
such an amount of variation in every direction that it becomes 
absolutely impossible to affix any set of characters to them that will 
lead to their future recognition. 
The sub-genus Aciculina, H. and A. Adams, is made up of 
varieties of one shell. The N. labiosa, J. Sowerby, in Wood’s 
Mollusca from the Crag, is simply a grooved form of the Philippine 
shell, N. maculata, A. Adams, and Professor E. Forbes was quite 
correct in the statement of its being a variety of the N. propinqua, 
J. Sow., or semistriata, Brocchi; the comparatively smooth forms 
pass into varieties of the former, and the flattened ribbed varieties 
into the latter shell. I do not consider the suggestion of Mr. Bell 
as being either impossible or even improbable, when he states that 
varieties of N. propinqua closely resemble, and may be considered 
to be, varieties of N. trivittata, Say. 
The nodules occur in three different ways, viz., as protuberances 
on the ribs, in the square spaces formed by the transverse and longitu- 
dinal lines, and at the junction where the lines cross each other. 
In the N. subspinosa, Lam., the tubercles are very prominent on 
the ribs, and some of the varieties have them produced to a point, 
hence the name ; the cross-grooves are rather close and waved, but 
we find other varieties of this shell without the nodules, their 
place being occupied by strong raised ribs ; others again have sharp 
