INTRODUCTION, 7 
sometimes most irregular, both in size and rotundity ; an instance 
of this occurs most conspicuously in the N. nodosa, Marr. 
The various changes that take place in the shelly matter 
deposited on the margin of the sutural canal is used by the 
conchologist as one of the characters for the discrimination of the 
different species; how far this can be relied upon as permanent 
may be inferred from the following remarks, commencing with the 
shells having the body-whorl gradually tapering into the penult 
without increasing in thickness at the sutures. Examples— 
N. rufula, Reeve, N. glans, Linn., varieties, N. Marratii, Smith, &e. ; 
thickened, and forming a sharp callous edge to the sutural canal 
(suture canaliculate)—N. spirata, A. Ad., N. laevigata, Marr., var., 
and N. canaliculata, Lam., &c.; tumid—as in N. tenia, Gmel., 
vars., N. Glans, Linn., vars., and N. picta, D’k’r., vars. ; the tumid 
band divided by groove-lines—N. glans, Linn., vars., N. coronata, 
Brug., 
folds—N. crispata, Marr. ; beaded or coronated—N. celata, A. Ad., 
var. N. variegata, A. Ad., and N. cremata, Hinds; strongly 
noduled — N. arcularia, Linn. very grade of difference 
and N. levigata, Marr., vars.; with close and numerous 
between two extremes in shells, in what are termed of the same 
species, may be traced thus: N. glans, Linn., presents examples of 
each of these changes in the series from one end to the other, and 
several other variable shells change in a similar way; and, as almost 
every shell has points of difference, and consequently varies, the 
shelly matter on the margin of the sutural canal becomes a doubtful 
character for specific distinction to be based upon. 
The nodules near the sutures are either flattened like the ribs, 
or raised into large tubercles, such as we find on the N. arcularia, 
Linn., and all the intermediate sizes may be found in the different 
varieties. If the shell be finely cancellated, the beads will be small 
and numerous ; but if the ribs be broad and the cancellation coarse, 
the beads will be large. The groove-lines forming the transverse 
sulci are situated at almost ‘every space on the shell, from the base 
to the apex. The ribs may extend from the suture to the base 
without having a single groove-line, or they may be intersected at 
regular intervals ; the line just below the suture in a shell before 
